Common oysters and mussels, like you'd get served at a seafood restaraunt, often live in brackish estuaries. They are intertidal species; I don't know how well they'd fare in constant submersion.
There is a family of oysters called "tree oysters" (Isognomonidae) that normally grows on mangrove roots. Maybe you could get someone who lives in the tropics to mail you a few.