Most laminates don't like a lot of water...is this a full bath, or half? DOes the manufacturer allow it in a full bath?
As noted, this kindof floor needs a quite flat surface. You could use self-leveling-cement. The stuff is fairly expensive, but for a small area would allow you to get the area perfectly flat.
Backer board (cbu) does not buy you anything unless using tile. The better thing to do is put down some plywood - it will reinforce the floor, and you can get it close to the exact height you want.