That still doesn't help very much when you are looking behind you trying to judge your opponent and the water during a nose down turn close to the deck.
I just came out of Florida again today and made sure to give the water a look and take a guess as to my height.
Even at 10,000 feet I was guessing within a couple hundred feet.
One doesn't need an altimeter unless the water is dead calm and glassy--and I would suspect gross estimates would still be easy.