if you install wood in a full bathroom ,the best product would be an engineered type wood floor,and no you should not install ceramic over ceramic...good luck

I was buying some teak for countertop edge recently, and the wood store people mentioned that White Oak is a decent water resistant wood. (Teak is better, but for a floor would run quite a bit of money--for a countertop edge it is almost affordable.)

I kind of question the wisdom of using it for a bathroom floor, however. Lots of moisture; shrinkage and expansion, etc. You'd probably really have to find the right finish for it.

Be ready to replace or sand alot. If there is alot of moisture, it will raise the grain of the wood.
Wood is not very practical in a FULL bath, just as carpet isn't either, but I have seen it.