Aang is the type to blaze new trails and seek out new exciting things! This is why he's walked up the beach a bit to explore, and found a private little cove hidden away, with tide pools and the like. Not far off is an underwater cave even. Being the animal-lover he is, he's been raiding tide pools, ever vigilant in using water bending to keep his presence from disturbing the critters within, and looking at all the little sea animals with great curiosity and marveling over their existence.

But then something out in the water catches his eye. No one is there to hear him, but he gasps excitedly, when he sees something glinting out in the water, rolling and twisting and jumping around:

"Seal-dolphins!"

Oh wait, looks like just-dolphins. Still!

He'd already been stripped to the waist, but the last of his clothes other than his orange underwear are flung off during his mad dash to the water, and before long, after a careful, gentle approach, anyone that comes along and finds his discarded clothes in the sand and looks out in the water to find their owner, will see a bald young man, happily swimming among a pod of dolphins, and occasionally hitching rides.

Rather than being upset or nervous around him, they seem to be enjoying playing with him, as if they can't even tell the difference that he's not one of them.

You can try to join him but make sure you move slooowly in the water, to not spook them off.