So, here's what I decided on for a solution in the hot tub nook (currently carpeted, and directly above Caleb's studio, which needs both silence and protection from water leak-through):
So much cool stuff here:
http://www.rubbercal.com/Comml_Matting.html I think the
Elephant Bark is the best option in terms of price, watersealability, recycled/eco-friendly/american-made, soundproofing. It'll be about $400 for the project, which is a lot cheaper and easier than tiling the entire room, and leaves less maintenance and noise worries (people on tile are just not quiet, and I hate dealing with grout maintenance).
So. When we've got the cash, that's the direction we'll go in that room, in case anyone was curious how it all came out.