Your water could be salt water and the enemy could be a freshwater fish. That'd make sense, at least from a realistic sort of standpoint. That or like, the temperature of your water spell is vastly different than the environment the fish enemy is used to, and it's introduced to that level of temperature too quickly and dies of shock.
Anyway I didn't use Bear Traps all that much. I don't quite have enough buttons for that. I should be getting a nice 5 button mouse soon, so I surely should be bringing traps into my play later on.