Generally a metal or vinyl framed window can be used in a bath enclosure provided the framing is protected. Tile works well in this application and can be used to cover all sides of the window opening. Even if you don't use tile in the bath, a marble or granite surround in the window can be made to look like it belongs. Of course you need to protect the other walls around the tub and use a shower curtain or door. Again, the tile works well in this application, or you might opt for a pre-manufactured surround being careful to seal around where the window opening is cut out of the surround.