Yes, I've used it as an adult. I swim for aerobic exercise 2-3 times per week. I usually take a few minutes to swim underwater for as much of a pool length as possible. Then whenever I swim with my kids, I do it 2 or 3 times. It is not dangerous and I'm not sure what benefit it has provided me. But heck, I'm going to continue it.
In the book "What A Great Idea!" by Charles Thompson, he refers to Yoshiro NakamMats, the creative genius who hold more than 2,300 patents (more than double Edison's). He was the guy who invented the floppy disk, compact disk and disc player, etc. The third and final part of his creative process is performed during held-breath underwater swimming. He created a plexiglas writing pad so he can stay underwater and record his ideas.
Win said he used it as a child all the time and claims it accounts for many positive effects throughout his life.
