enviroko said:
... Is there something that I can seal it with so that water will not be a problem with it?
Plastic sheet underneath Cement Backer Unit (CBU) cement board. Make the plastic longer than the CBU so it hangs lower, so water drips down into the tub's tiling flange. Cut the CBU so short that it leaves a little strip of air (a gap) behind the last row of tiles at the bottom instead of allowing it to touch the tub.
You have asked the right question, at the right time.
To help you to confirm this in your mind, search on key words
mold, greenboard, "paper facing" (both words together with quotes). You will see that no mold resistant product can resist mold for long. The term is not helpful to owners, builders, occupants; it helps sales, not safety.
If you want to leave the product on the walls, you can add Kerdi on top. Or Wedi. Or Redgard or Mapei. This will make your space smaller by 1/4" - 1/2" so factor that in when you tile.
David