A moisture barrier is all you need to keep water out of the water cavity. However, you'll need more extensive waterproofing for horizontal surfaces if you want to prevent a shower from leaking. If there is no plastic or roofing felt behind it, then a liquid membrane must be applied. In fact, Durock is water resistant, which means it doesn't get damaged by water.
But it's not a barrier to water at all. Durock does not deteriorate in the presence of water, but it does not prevent water from absorbing it. Concrete or wonder board is a cement-based product designed as a cladding material for walls and floors. It is used as a submaterial behind tiles in the shower, bathroom or kitchen.
The cement board comes in sheets, usually 3'x5′, and is secured to the heels with special screws. There is a misconception that cement board is fully waterproof because it is designed for wet areas, but it is not. When installed, cement board is waterproof, not waterproof. This means that the cement board needs waterproofing.
Before installing any tile, all joints, cracks, screws and corners must be properly sealed and waterproofed. Some builders even recommend sealing all large flat areas as well. This ensures that no water can penetrate through the cement board and into the frame, which can eventually cause rot, mold and deterioration. Custom Building Products requires two coats of its Redgard liquid waterproofing on all waterproof surfaces.
Waterproofing can serve as a moisture barrier, but a moisture barrier does not necessarily qualify as a waterproofing barrier.