Here's 2 I can think of off of the bat:
1: shark pool: send one blue into the pool at a time to fight the reds (already in the pool) with melee weapons. Doesn't have to be done to completion.
2: jump in: the reds stand by the pool side. You give the reds a key word to jump into the pool with (like sausage). you say a sentence, and when you say the word, the reds must jump into the pool. You can decide beforehand which red dies, but usually, you make the last one in kos.
A game that would clearly favor one class, like a swimming race, or who can stay underwater longest, would not be a good idea.