Agreed Chuck, though I venture a guess that you were doing it in a large flying boat. That would offer a bit more useful altitude for the "1/2 wingspan" often considered ideal for ground effect. And, I expect that the ocean is rarely glassy, so visually determining your altitude was a bit more certain.
In a small flying boat, I have found that I have to have the keel just a few feet up to experience any ground effect, and that's too low for any sane person to fly a landplane over water at cruise speed!
In any case, if the water is or suddenly becomes glassy, determining your altitude visually will suddenly become impossible, and immediate climb is the only safe action.