I wouldn't use FlexWatt, either. Use something like this:
http://www.wayfair.com/WarmlyYours-T...FcVrKgodYipdCw

It's designed for that purpose, it's sturdy, and it's not prohibitively expensive when you're talking about tiling a room.

If the room is well-insulated, you might not need any additional supplemental heating, but you should put in 2 separate pieces of the floor heat, then, so you can set different temperatures on different sides of the room. I would check air temperatures after you have the floor heat installed, and see if you need any heat panels.

For the walls, you may want to consider using cement board. Green board is moisture-resistant, but cement board is moisture-proof and won't mold. Don't forget about the ceiling. Standard sheet rock isn't designed for constant high humidity.

Are you going to install a floor drain? (This idea is one I've put a bit of thought into as well, lol).