I think this might fit humans who live by the sea much more than dwarves, but I'd really like to make the ocean and seaside settlements more interesting...Here's some ideas:
bivalves/immobile vermin are already in - there's mussels and oysters in the raws, although I never saw them in game. However, a small addition could be that, when you process oysters, you have a VERY small chance of getting a pearl. There would be several different kinds of pearls, and it would be considered a gem, although you would not be able to cut it. You could decorate stuff with it of course.
It would also be interesting to have coral in tropical oceans. Would be somewhat hard to get, but it would have some worth as a trading good, and it could be carved/crafted - not sure what skill it should use: Bone, Stone or Wood crafting?
Algae as a plant that can be gathered and used. According to wiki algaes are quite useful. they could be used as fertilizer, fuel, food and dyes!
This is slightly offtopic, but it would be amazing to having underwater entities, perhaps not as fleshed out as aboveground ones.
Imagine being able to trade goods with the Merfolk - you could sell them metal weapons(hard to forge stuff underwater)in exchange for crafts and seafood! However, you could also be attacked by humanoids emerging from the waters, a bit like Deep Ones.
Merfolk could also ask for some kind of treaty against overfishing, but it would be quite different from the tree treaty the Elves give - for one, you can't control fishing as well as you control treecutting. Actually, I'm not sure this is that realistic an idea, since overfishing an ocean seems hard to do using non-modern methods...
[ June 14, 2008: Message edited by: Zonk ]