A child born by a violent water birth, Resulting in the baby shooting across the tub and hitting its head in the side causing it to become a little dumber.
Another great example of a water baby would be Percy Jackson from the movie "Percy Jackson and the Olympians". Percy says that whenever he is in the water, he can think more clearly on things.
"I had a terrible cramp this morning, so I took a shower. Wow, did I feel GREAT afterwards! It's good to be a water baby."
"That Percy Jackson character is a total water baby version of Zac Effron."