What's amazing to a newcomer researching ocean health is not how much is known, but how incomplete knowledge is compared with the potential seriousness of what we are doing.
For instance, with or without a rise in water temperature, a slight increase in ocean acidity is tough on coral reefs, and it inhibits calcification by phytoplankton. Reefs might or might not adapt and recover in some form, but not if acidification keeps running ahead of their adaptation pace. Reefs are the bottom of the food chain near shore in tropical zones; without them fish catch is greatly reduced.
O2 and CO2 exchange with the atmosphere takes place at the ocean's surface layer. The chemistry in that layer is different; it's different in many layers going on down, too, but the surface is critical. For example, some fish eggs lie at the surface to spawn. The importance of that is quickly grasped, but a bigger question is how much that is critical to this huge cradle of life impacts human welfare, but we don't know about it -- or worse, we think we do, but really don't. (Just doing a Google search of "ocean surface microlayer" is apt to give you a jolt.)
The plastic "nurdles" in ocean gyres are starting make it into mainstream news now and then. There are at least 5 such gyres; the biggest is the Pacific Gyre. The size of these things is hard to even roughly estimate, so the Pacific Gyre is somewhere between the size of Texas and the size of Africa, an expanse coated with nurdle scum, tiny particles of plastic waiting to biodegenerate in a century or two. Nurdles may not be noticed from shipboard; only the large pieces of junk are. About 80% of this stuff is estimated to originate from land sources; the rest from dumping at sea.
So keep going, and do your own checking from reliable sources.