Teak flooring in the shower, good idea or bad?
The teak wood is not the actual floor of the shower it is just a layer on top of the waterproof substrait or tile.  There is no wood that has properties to form a shower subfloor - it holds too much water, expands and contracts too much, and rots.  Not to mention sanitary issues.

So can you put teak in a shower.  Sure.  You can stand on top of whatever you want as long as the underneath part is waterproof and drains.  However you are just adding another (major) thing to clean.