Marco Polo is common.
Rainbow: is when one person is out of the pool and the people in the pool pick one color of the rainbow. The person out of the pool faces there back away from the people. They shout out the colors slowly. They can only turn around after they say three colors. Then when someones color is shouted, they have to sneek away quietly and get to the other end before the person who is it finds out they snuck off tries and catch them.
Diving Competition: If you have a diving board or not this is a fun game. You have to have at least 4 people or honest people to play. You go on the side of your pool (or diving board) and say a trick that you are about to do off the diving board, you do the trick and your judges (friends or family) give you a rating our of 10. You do three rounds for every person that is playing.
Twirl-A-Whirl: Get in the pool and do as many forward summersaults or backward summersaults in the pool without taking a breathe or stopping. Who ever does the most summersaults wins.
Hold Your Breathe: Go against as many people as you want. You all go in the water at the same time and hold your breathe. the one that stays in the water the longest t (or the last person to break to surface wins.)
Race: Time yourself or race against others to see who is the fastest swimmer, or who can get the best time.
Make me laugh: 2 people at a time go in the under the water and stare at each others eyes. The person thats it tries to make the other one laugh. As soon as the other persons starts to blow bubbles to the surface or goes to the surface, they are then it. If you cant make them laugh and the brake surface, you have to keep trying to make them laugh until they finally give. (If they go to the surface you have to go up, then you go back down together and try again.) The only thing you cant do is touch them to make them laugh, thats cheating.