At some point in a water polo game you have found yourself held and unable to move. Here is a video that provides a remarkably clear view of some ways to get out of holds and turn them to your advantage. Ignore the ridiculous bikini! Clearly NOT a water polo player.
If you want to implement some of these techniques, you will need to PRACTICE! But, it does not need to be in the water. Practice at home on an unsuspecting parent or sibling until you can do it without thinking. Then try it in practice with a teammate. Then you can look for your opportunity in a game setting!