When you dive, It puts a timer for a few ticks (5-6?) on you that prevents you from diving again. It's really only a pain when you're in places where you go from land to water to land to water, etc., because you have to take breaks.

It also destroys your morale, but that's not the worst thing ever.

I've played felar and arials that went to Kuo-Toa, for example, quite a bit.

If play to explore underwater a lot, I'd avoid races with hydrophobia. It's annoying, and it'll get you killed in some places where the mobs track you while you sit there and wait for the timer to wear off so you can run away.