I tried a flotation tank twice this last summer. I was especially interested since they use magnesium salts to provide the dense solution that makes one float. Magnesium can be absorbed through the skin (varies from person to person, though) so I was hopeful that either that or the relaxation would provide some relief.

The short story, though, is that while the experience provided some relief the first time, I didn't find it anywhere near as relaxing the second time. My sleep apnea woke me up with a start and I couldn't get re-relaxed. Although I have a mild case, I don't think my apnea will let me enjoy that form of relaxation.

Reminds me of an old Calvin and Hobbes cartoon where Hobbs says he was "meditating and meditating... and then I woke up!"

Try it, though, see how it works for you.