I have a friend who shoots amazing macro stuff using this technique... but it's in a studio (his wife apparently lets him bring bugs and spiders in to the house!). I routinely 'borrow" his images for my desktop, but they usually give me nightmares. ;-)
He and I have never discussed his technique but he sometimes posts images of his studio and his tripod is a serious piece of kit, with a ball head on it that probably cost $1000. I don't know most of us would want to take such a thing into the ocean.
I suppose if you could find a bare sand patch to lie on and then use a mini-tripod, you could potentially get something. It would certainly be fun to try. Personally, I find shooting with a wet diopter on a 105 more than enough to manage. But then I am more a shipwreck guy. Macro kinda creeps me out... all those little squinty critters that want to burrow into my brain. But I digress... ;-)