A friend of mine used to fly low enough to get wave-lift, which supposedly cut fuel consumption. If my understanding is correct, this means flying at less than half the wingspan above the water,
Yes, I was training this yesterday, and there is a favourable affect. BUT, we do it over a surface with enough texture, that you can constantly assure your altitude, and in an aircraft/surface combination which will assure a safe landing if the engine quits.
Yesterday's objective was a 5 mile flight, with the keel 2 - 4 feet off the water. I'll occasionally fly my 150 with the wheels 3 feet or so off the ice of a frozen lake (upon which I already know I could land if necessary). I'll pick up 3 - 5 MPH doing that.
Doing that over glassy water, or unbroken snow is suicidal.