From an advice column in the San Diego Reader:
"Hey, Matt: Why does all seafood smell the same, even though it comes from such different kinds of animals (mollusks, arthropods, vertebrates)? Okay, so maybe it doesn’t all smell exactly the same, but similar. The only thing all those creatures have in common is that they live in seawater and that “seafood smell” does not smell like seawater."
The columnist, Matthew Alice, responds with a good overview of why, exactly, seafood smells so unique.