OceansD1 cover 71% of the planet and contain 97% of all water on Earth as they average 3.8 Km in depth. They are rising in levelP, warmingD2 b, acidifyingD3 b, diluting many other types of pollutionw L, and most fisheries are subject to unsustainable fishing practices at nearly every level of the food chain.  Less than 1.6% of the oceans area is protectedL Impact Map.
Fishery and ecosystem collapseL along with the destruction of most coral reefsL L is forecast to happen before 2050L L w. The abundance of large fish, ocean going mammals and reptile species has already suffered large declinesD4. Even the abundance of plankton is in decline.  

In many cases, species tens of millions of years old, some with longer life spansw than humans, have experienced a 90%+ decline in abundance e.g. blue fin tunaL, most sharkL, whaleL, and turtleL species. 50% of sea bird species are in decline with 28% classified as endangered with extinction. The number of threatened fish species has doubled in 10 years.
The vast oceans are home to most species on the planet, and the source of more than half our oxygenD5. They also absorb an additional amount of CO2 equivalent to 25% to 29% of our annual CO2 emissions, and 90% of the heat Earth receives from the SunD6 b. Ultimately all CO2 is cycled through the oceans and deposited on its floor, or re-emitted to the atmosphere and the living carbon cycleD7 b.
The oceans have a far larger carrying capacity for both carbon and heat than the land surface and atmosphere combined. This is down to waters properties and the fact that the oceans average 4 kilometers in depth. So it's no wonder that it takes time for the land surface warming for a given level of atmospheric CO2 to feed through. We can tell the oceans are warming as there is a network of thousands of Argo FloatsL around the world. A warming ocean means the Earth is out of Energy BalanceL and more surface warming is in the pipeline.
The oceans surface waters average temperature in 1880 was 15.9oC [+/- 0.1oC]. It has since risen 3.7% or to 0.6oC to 16.5oC. This surface water warming is amplified in polar regions. Polar regions had an average temperatures ranging from 2oC to 5oC in 1880, and have since warmed by 50% or 1oC to 2.5oC on average for key periods of the year. Warmer polar waters are a key mechanism in accelerating ice sheet melt.
Warmer oceans expand in volume causing sea level riseD8 b and promote ice melt as it undermines the ice shelves, which buttress the glaciers entry to the sea, accelerating further sea level riseD9 b. They also erode sea ice from underneath causing it to thin and shrink in surface area. Once the sea ice is gone the reflectivity (Albedo)L w of the Earth's surface decreases substantially and more heat is absorbed by the darker ocean, strengthening the above processesm P.          
Warmer waters also hold less O2 and are prone to water stratification which causes less nutrients to up well from the deepw.  Consequently the size, frequency and duration of Plankton Bloomsw decline, and with it, the productivity of fisheries and average fish size (-14% to -24% link). For example in the North Sea the dominant form of zoo-plankton has declined in abundance by 70% since the 1960's as waters have warmedL. In the open ocean phyto-plankton has declined by 40% since 1950. The capacity of the Oceans to absorb CO2 declines with reduced plankton abundanceL. This has not yet happened as coastal plankton blooms have increased in response to eutrophication caused by fertilizer run-off from agriculature and cities.
Warming in coastal areas when combined with excessive EutrophicationD10 creates Dead ZonesD11 w L. Dead Zones are areas with insufficient oxygen for fish life and frequently occur even at current atmospheric levels of CO2 and warming. Coastal zones are the most productive fisheries accounting for more than 50% of fish caught. The coastal fisheries rely upon coral reefs, mangrove forests, marshes and sea grasses as nursery areas for juvenile fish.
Warmer waters also cause coral bleachingw events from surface water heat waves. These excessive transient temperatures force the coral to expel the algae which gives it its color and is necessary for reef survivalL. In warmer oceans the optimal temperature zones of habitat shift, leading to species migrating, and disrupting the ecological balance in these areas. A 2oC global average warming is certain to create conditions that will eliminate all corals [link]. A third of the oceans species life cycles are directly connected to the coral reefs and their health.
"Our projections suggest that most coral reefs will experience extensive degradation over the next few decades given the present behaviour of corals to thermal stress. To protect at least 50% of the coral reef cells, global mean temperature change would have to be limited to 1.2 °C (1.1–1.4 °C), especially given the lack of evidence that corals can evolve significantly on decadal timescales and under continually escalating thermal stress" [Link].
The ocean's surface waters [top 300 to 400 meters] also hold 30% of industrial CO2 emissions since 1800 [over 330 billion tons of CO2, equivalent to 42ppm atmospheric CO2]. Yet we know that at a certain point of warming, oceans cease to absorb CO2 and rather emit itL. This is a climate feedback mechanism is why North America and Europe are presently not under 2 km of ice.
The pHD12 b w of the surface waters of the ocean is falling in the top 300 meters, where 95% of ocean life isL dl, due to fossil fuel CO2 emissions. These waters are absorbing more incremental CO2 than the thermohaline currents can keep up with. They take up to 1,000 years to over-turn to provide the chemical refreshment from the deep to maintain pH L.
This has resulted in the surface waters average alkalinity decreasing by 30% since pre industrialization: a process labeled ocean acidificationw. The current rate of acidification is more than 10 times the velocity seen in the last great oceanic extinction event 55 million years ago, 100 times that of the last 20 million yearsD13. Each of these afore-mentioned changes in ocean pH took thousands to millions of years to occur.
Under a business as usual emission scenario [850ppm CO2 by 2100] ocean acidity is projected to increase to well over 100%  by 2100D14 b and ultimately much more, creating conditions unprecedented on the planet in 55 million years. Lower emission pathways create a similar effect once possible natural feedback CO2 projections are included.
The projected ocean pH levels under business as usual, flat or even moderately declining CO2 emissions all eventually create conditions where the oceans ability to draw atmospheric CO2 down from the atmosphere is severely compromised, and are even enough to dissolve the shells of organisms and calcium carbonate deposits on the ocean floor. Even the 2oC senario using the 450ppm overshoot methodology severly compromises the oceans pH.
Like atmospheric CO2 concentrations, and the resultant global warming, Ocean “acidification” is irreversible over many thousands of years as it is also a cumulative CO2 problem.
Acidification threatens the base of the oceanic food chain as it hinders the growth of some forms of planktonD15 w, PteropodsW , juvenile creatures and other life formsD16 which use forms of calcium carbonate to make their shells,D17 e.g. coralsL. They require a specific saturation level of calcite and aragonite in the water to grow properly. Continued acidification would reduce the saturation levels further, to concentrations where growth is difficult, meaning a decline in their abundance, and that of other interdependent species. [Aragonite saturation projections 30s      ]
Acidification [& stratification from warming] adversely affects the juveniles of most fish species (e.g. sardines), and molluscs, as these are entirely dependent on plankton for food. Larger fish, bird and mammal species     whose prey feed on plankton, or require calcite or aragonite to grow shells would also be severely affected     . There are several other numerous damaging side effects of increased CO2 in the oceans.dl
The mechanism is first felt strongest in the oceans at Earth's Poles, as cold water can hold more CO2. Cold waters are where the richest feeding grounds are, as the plankton blooms are largest due to nutrients upwelling from the deep ocean currents.
It has been found that the shell thickness of species of Pteropods [sea snail] in polar regions have already fallen by 35% since 1800D18. This creature occurs at high densities and is a key protein food source for fish. Small declines in their abundance have large follow on implications up the food chain (Moy et al, 2009).
Experimental results also show that Krill eggs have decreasing survival rates as the ocean pH levels rise. Krill is a keystone speciesw. It's decline would be a catastrophe D19     .
These are the creatures     upon which fish, seals, penguins, dolphins, sea birds and whales ultimately depend upon for their food.            
Even the proposed international targetD20 b of peaking atmospheric CO2 at 450ppm to limit warming to 2oC in the 21st Century would eliminate the majority of habitat for coralsD21, the home of 30% of oceanic species, and cause the certain decline of the base of both the Arctic and Antarctic food chains...
Indeed, the major food generating ecosystems at the base of the oceanic food chains and corals have already experienced substantial decline Biological Pump.
Open ocean Phytoplanktonw blooms have decreased by well over 30% since 1950L indirectly due to warming  and possibly acidificationD22 b c d e.             
Healthy Coral Reefs have already declined in abundance by up to 50% in the last 50 years through a combination of exploitation, pollution, bleaching from warmingL dl w, and acidification. Corals play a crucial role as a nursery for the juveniles of a third of marine life, and they are being lost at rates similar to virgin rainforest.
In addition to the above, the important coastal erosive break-waters, carbon sinks and juvenile fish habitats created by salt water Mangrove forests, marshes and sea grasses have lost 40% of their extent through development and are presently declining at rates of 2% to 7% per yearL...ironically often for fish farming.
Overlaid on the above dynamics is the problem of over fishingw. Global fishing fleet has signifiacant over capacity of 200%. When combined with modern technology, poorly regulated waters, and practices where 30% of catch is thrown back dead, and the sea floor ploughed with steel, it is unsurprising that most fisheries are in steepfishery decline [particularly for long lived and larger species]. 

And then there is the industrial and extractive pollution [oil & gas]... It is absolutely clear that Nature as it expresses itself in the ocean is in trouble. And that’s where life first began...
Both ocean warming and acidification are caused by CO2 emissions. Ocean Acidification is a predictable chemical process, independent of global average temperature. It alone is a reason to eliminate fossil fuel CO2 emissions.
The extinction of many thousands of oceanic species, and destruction of the ocean's ecosystems and fisheries will occur if the 350ppmD23 b Atmospheric CO2 goal, and protected areasL are not implemented. It would take hundreds of thousands of years for the diversity of the oceans to recover.

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