Can't talk underwater. Or can you?

[Big D]

I started a new FFX game (forgot to give Oaka his money on my last playthrough, so have to start over), and I noticed that Tidus says "Whoa" underwater during the sequence with Geosgaeno. Yet everywhere else in the game, characters communicate with gestures and hand signals underwater (and the hilarious soundless underwater laugh). The question is, why, when they can speak underwater with no distortion?

You can shout "whoa" or whatever underwater quite easily. Also, Wakka yells "Yuna, we'll save you!" or something during the Moonflow incident, but it's very muffled by the water.

Talking underwater is a waste of valuable oxygen, especially when you've got to stay under for several minutes at a time to be a pro blitzballer.