For some reason, all of the underwater mobs added by Calamity are immune to summon weapons. Minions ignore Calamity's underwater mobs as if they weren't there, and if using a non-minion summon weapon (such as the Death Wind from Spirit Mod), the attacks don't even hit unless its a melee-like strike. EG: direct attacks with the Death Wind will hit, but the scythes thrown by it do nothing.
EDIT: Okay, not all, just most it seems like. MOBs which are seemingly immune to summon weapons include Giant Clam, Moray Eel, Ghost Bell, and Eutrophic Ray.
EDIT: Add to the list: Aquatic Scourge, Viper Fish, Devil Fish