To my knowledge, all of them are predatory (like with other kinds of snails, there may be a random oddball exception or two, but I certainly wouldn't expect it).
The snail pictured appears to be the Lettered Olive Snail, Oliva sayana, which reportedly feeds on bivalves (clams, oysters, mussels, etc.), pods/small crustaceans, and worms.
Edit: And detritus/dead animals of other varieties too.