Nothing you SEE - in fact hell of a lot happens down there that effects us; just in ways so indirect we don't even realize it. Undersea volcanos and eruptions forming new land masses (however small) causing the tectonic plates to shift (earthquake anyone?). Overfishing/hunting/harvesting can lead to extreme losses of key species allowing some species' (like whatever the key species fed on) populations to explode without check (which leads to even more problems) and other species (like predatory ones) to search for alternate food sources (one shark to another: I hear the coastlines are filled with clumsy, bad-swimming mammals in brightly colored clothing this time of year). Not to mention all the damage done by pollution, illegal dumping of wastes, and the pumping of oil. In short, quite a lot happens underwater and sooner or later the problems will come back to bite you in the rear. The reason they don't show these types of issues in the comics is simply because they are indirect in effecting the human race a.k.a. readers have short attention spans and the writers risk boring them by taking the time to show the numerous steps detailing exactly how the ocean's problems will come back to haunt the human race.
Aquaman could do quite a bit if he chose to - I'd like to see how many fishing boats survive being rammed by whales (the largest creatures on Earth), how many subs wouldn't implode after being dragged to the sea's bottom by giant squids, and how many people would continue to visit the coast if every manner of sea life from jelly fish to crabs to seals to sharks attacked them if they so much set one toe in the water.