I rearranged the main theme of SuperTux for (entirely) underwater levels (FYI: to make one, create an extra layer, give it a negative z index, make it solid and fill it entirely with water tiles) - listen here:
http://rm.cloudns.org/~xonotic/temp/supertux-water.ogg
See all data files (including .music, .mid and .rg file - and yes, it loops seamlessly when using the .music file) here:
https://github.com/divVerent/divTrololo ... lolo/music
Enjoy!
If this is to be considered for inclusion in SuperTux, I am of course okay with that - license is the same as for the SuperTux theme itself (GPLv2+ or CC-BY-SA), which is forced given it's a derived work, so it's not like I could stop anyone.
Finally, to give credit where credit is due, the idea to do such a style transfer for underwater music is from Super Mario World.