The short nothingness of human existence and our perturbations to the natural world won't mean much to Earth in a few billion years, but the damage done to the ocean is definitely not reversible on human timescales. Pollution is the problem and there are a several sources that are especially bad, including pollution we can see (like sewage and trash) and carbon dioxide that dissolves into the ocean from the atmosphere, making the water more acidic.

You should watch this video about plastics in the ocean, it's pretty horrifying to see how bad the plastics problem is:

And in this video about ocean acidification, you can learn a bit about how carbon dioxide in the atmosphere affects ocean life:

We can help minimize further destruction of the ocean by not being wasteful with plastics (which you might know are made from fossil fuels) and by decreasing our energy consumption. Unfortunately, a lot of people just don't care about our environment as much as you seem to. It's great if you can peer-pressure your friends into not buying little plastic things they're just going to throw away or by riding your bike places instead of driving, but in order to get everyone on board, there needs to be very high taxes on fossil fuels so that it's just not convenient to drive down to the store in a gas-guzzling vehicle to purchase disposable plastic plates. I believe high taxes will help force everyone to do their part.

The answer is only partially known. Theoretically, any species that is not extinct can recover, and there are very few documented cases of actual extinction caused by over fishing (although there are some). That said, a lot of species are endangered, and ecological shifts caused by changes in species dominance might be harder to overthrow. For example, adult cod are predators of the predators of juvenile cod, so now that the adult cod have been over fished, the juvenile cod are being eaten by formerly unimportant predators.

Your second questions are political, not scientific. The obvious thing from a policy standpoint is "over fishing needs to stop", but there is of course disagreement on what the best way to do that is, especially considering that literally too many people depend upon fishing for their livelihoods.