I would guess on a micro level the best thing you can do is recycle, reducing the amount of refuse that ends up in landfills and often times out at sea. A lot of it from coastal areas ends up in the Great Pacific Garbage Patch, and it's not like storing it on land is awesome either.

The problem really is that even decently sized macro action can be largely negated by economic freeriders. Unless some body like the UN or US sets out to enforce universal cooperation with agreements on pollution, or fishing, whole states that don't follow the regulations will continue to take advantage. Witness the failed Kyoto Protocol, which had 191/192 nations in agreement, but the very worst polluters (read: the US) who refused to be party to it.

As the other person who replied only really addressed pollution, I'll add that you should stop eating fish. And if that's asking too much, catch it yourself. And if that's asking too much, there are definitely operators out there who are concerned about sustainability, you should insist on purchasing your fish from independently certified sustainable sources