This is really only true on Arch maps, though. On other water-based maps, there is usually enough land tiles that can be improved and worked. In fact, I think that you would be surprised how few coastal tiles you will actually work. Sure you might have a 1 or 2-land tile island with 3 Seafood here and there, but for the most part, on maps that DONT use "tiny islands", you wont work nearly as many coastal tiles as you imagine. Between whipping population as your main source of production and trying to work resources + cottages, (or running Specialists), you are still in a situation where a non-seafood coastal tile is sort of a "last resort".
Now, I still think FIN is very powerful on water maps because of those tiles, and because of the vast amount of seafood tiles you will work (which is the same reason I think Colossus is worth it on those maps), but I have played many many variations of "water maps" and you just never seem to work those coastal tiles as much as you think you will.
They sure look pretty in the city screen, but what is prettier is having the Trade Routes require you to scroll to see them all.