Ground effect height is a function of the size of the wing. I'm guessing these are going to have to be massive to be of any use, otherwise you'd be so close to the water that you couldn't "fly" in choppy water, and with a strong headwind you'll need even more margin because I'm guessing that waves moving with the wind are going to reduce the ground effect.

According to the article the ground-effect ceiling is around half the wingspan of 20m.