Speaking of fish and fishing, sharkskin is extremely useful. Sharkskin and manta skin (not true fish, but close enough for this conversation)can both be made into leather, also, and shark teeth would make fine tools. So it's not just the meat that fishing could provide.
(Basking) shark liver was also prized by the Vikings, and cod-liver oil was seen as a medicine, so if we ever get butchering of organ-meat, fish could be included.
Some fish are even hunted more for their fat/oil than anything else. Whales were hunted for their oil, bones/teeth, and ambergris, as well as their meat.
Our dwarfs might occasionally go whaling with harpoons, if spear-fishing comes in. And whales and dolphins do occasionaly beach themselves, making for a lot of almost work-free meat, if you're in the right place at the right time to butcher it quickly and get it preserved somehow.
And, if we get better fishing, we really ought to be getting food-preservation too, because most fish back in the middle ages (and it was really just mostly cod that they fished) was smoked, salted, or pickled.
Along with fish/shellfish, we might also pull seaweed, driftwood (good for carving), ambergris (whales occasionally cough it up, and then it floats on the water, eventually being discoverable on beaches), and meerschaum (another floating material, that is carved and commonly made into fine smoking-pipes). Sea salt could also be harvested.
Also, dwarfs might go ice-fishing in winter, or in cold climates. They'd punch a hole in the ice with a sharp object (pick's fine) and then fish through that. That would be a great way to provide a little more protein and fat in your Fortress's winter diet. Additionally, it would give dwarfs feeling less social a good excuse to get away from everybody and drink hard liquor, while getting breakfast/doing something productive.