Hey Ralph,
Farther north on the East coast where I am, the water is too murky to effectively use this technique, so I don't do it here. However, "The Bulgarian" of South Florida and many who detect with him effectively use this method all the time. I was told that by having your head under water you are not distracted by all the noise of people, surf, etc and you can concentrate better to listen for those small whisper sounds that mean deep gold. By being under water, you also do not have the distracting sounds from your detector as the control box is splashed by waves. One individual I now who is about 5'-7" wears 30 pounds of lead weight to keep him under water and detects using his snorkel in water as shallow as 15" deep. On a shallow target he will just fan the bottom rather than use his scoop. This individual has been detecting this way for many years and says that you have to build up to it, since you tend to use different muscles snorkeling than you do just walking and swinging. Hope this helps. Nothing to loose by giving it a try. HH, Ralph Peters