Doing a diving race can be a really fun addition to your pool party. The prerequisite is that you have a deep enough swimming pool. If it's too shallow to dive in, then this game is out of the question.
You'll need to either form teams of two or appoint a neutral "judge". The "judge" or the teammate throws an object into the swimming pool and two contestants dive in to get it. The first one to come out with the object in his hand, wins!
Alternatively, you could also throw two objects in, one for each, to avoid any injuries from fights over the who gets it.
You could also throw in a bunch of object of two different colors. Each contestant gets one color. Whoever manages to fish out all his objects first, wins!