It's hard because the plural form of "fish" is usually "fish." I think we allow "fishes" when "fish" is ambiguous, like my example. Maybe we should just change to "fishes" all the time :)
'Swimming with the fishes' means you must have wound up on the wrong side of a boat in a diverse body of water. It would be a shame to have to 'swim with the fish'. So ambiguous.
Edit: late for the party. "Deeres" and "sheepes" follow the same pattern, I presume.