"When these people enter the water, not only can they not use their powers, they have trouble moving their bodies at all. They might be able to struggle a bit, but it wouldn't do much good. [i]That's if their entire bodies are submerged in the "sea." With less than half the body or just the limbs, it gets easier.[/i]"

I take this to mean if they are submerged, their powers go away. Partial submersion means a weakening of their powers and strength, which grows and wanes with how much is submerged. Ie just ankle-deep mean a small decrease.

So, yeah it does affect their powers but she would still be able to ride a wave.