Try an eggbeater kick

Commonly used to tread water, and stay in a vertical position with your head and shoulders above water. Start with your knees bent and your legs slightly wider than hip-width apart. Then "pedal" each leg as you would on a bike, only they'll go in opposite directions: while one leg pedals "forward," the other leg should pedal "backward." This one takes some practice to get used to, but it's handy for "resting" when your feet can't touch bottom.