Cement board isn't destroyed by water, but it is by no means waterproof. A properly built shower can be used even before the tile is up and not leak...the tile is the wear/finish layer, NOT the waterproofing layer. Having a wooden shower bench inside of the shower is just asking for problems. You have to waterproof OVER it so it is functionally OUTSIDE the shower (like the studs), if you want it to last (and remember, CBU is NOT waterproof). There are various ways to do this: sheet membranes or paint/roll/trowel on waterproofing systems. It must be done properly and not be haphazard. If you try to wing it, it will probably fail. ALso, keep in mind that the top surface of the bench that is waterproofed, must slope towards the drain and can't be level or it will pool water and fail. At the least, water will keep the grout dark and promote mold growth. The slope is critical as is the waterproofing. There are a couple of industry standards that describe ways to do this correctly; one of them is the TCNA handbook (Tile Council of North America). They are not all that DIY'er friendly, but they do describe various approved, tested methods on how to do things in tile that work. The handbook does not mention product brands, only methods, so you then have to choose a method and then the product to use.