American English at State

The plural of fish is usually fish, but fishes has a few uses. In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species.

Fishes, with an apostrophe, also serves as the plural possessive of fish—for example, the fishes’ scales were yellow. And of course fishes is the present-progressive verb (e.g., she fishes in the river).
Fishes also appears in the cinematic gangster idiom sleep with the fishes, used to indicate that someone has been whacked (and perhaps given a water burial).