An old block heater will work, plus they are made to go under water. Joe
Good idea, but bad idea too.
Only the part that is in the block is under water. You don't want to just throw the whole thing in and have the wires that are on the outside of the block short, or fry you when you touch it....
May have to make something to be able to use a block heater safely, to keep the wire side dry..