Doesn't sound like real aggression, but disclaimer: I've never seen him in person. However, if he were really aggressive, he'd have attacked and bitten somebody by now. His body would not be floppy and bouncy. If he's pouncing back and forth while growling, he's probably soliciting play. If he were biting out of aggression, his body would be very tense and rigid. He would not move quick and "floppy". A bouncing dog, pouncing back and forth around the pool, has loose and calm muscles. An aggressive dog has very tense, rigid muscles. The aggressive dog has entered a state of mind where he cannot control his actions. He'd be in a "strike" mode, like a cobra.

Honestly, it sounds like your dog is just being a very playful, mouthy puppy. It sounds like you might be reinforcing his inappropriate play behavior. Instead, when he barks, ignore him. If his nose is up your butt while you're trying to jump into the pool, stop moving and wait for him to leave you alone. You can also give him a chew toy to play with or confine him. If you're on the diving board and he's going after you, stand still as a statue and wait for him to leave you alone. If you hold out a hand and he mouths it, hold it there. He'll lose interest very quickly. You can repeat this many times so that he learns to ignore your hand. It's most important to give him alternative behaviors. Since he doesn't know how to control himself at the pool, teach him appropriate commands like a long "down/stay" and then give him a stuffed Kong.