It is true that water will penetrate grout. Tile is also not waterproof. That is why you need to use waterproofing of some sort in the whole shower, not just the bench. You can use plastic behind the substrate (cement backer board or mud, but never green drywall). Or you can use something on top of the substrate, such as a membrane like kerdi or a liquid like hydroban. If you use kerdi, you can use regular drywall instead of backer board.

We did our shower surround in corian and made the bench out of corian to match...open underneath with a waterproof light that shines out from under the bench for ambient lighting while steaming. Had to create a frame for the corian as by itself it is not structural. Corian matches the corian sink and counters we used in the bath...Egyptian copper.