Helium filled balloons tied to weights in pool. If you want to create the illusion of floating balloons use fishing line which is virtually invisible. (IDEA- Could also ADD some glow sticks so that the balloons GLOW over the Pool at NIGHT!
Omg, love to do this for Hayden's birthday party. For the pool party! Balloons floating in it would look cool and if they had a little water in them they would probably float well, be throw-able and looks soooo pretty and colorful and fun!