I usually use the same wave measuring device for sailing that I use surfing, my body. I am 6'. When I surf, I look back at the wave as I make a bottom turn in the trough. If the crest of the wave is at my head, its about six feet. Thats a very general rule, there are lots of variables. In the cockpit of my boat, I am about two feet (rough estimate) above the water. When you are in the trough of a wave, standing up can give you a rough idea of wave height compared with your height. If the crest of the wave is not at my chest height, (assuming I am flat and not on an incline) the waves are not six feet. Anyway just a rough estimate.
Great men always have too much sail up. - Christopher Buckley
Vaya con Dios