Parts of a bathroom beyond the shower enclosure get wet. The floor perimeter where water splashes out from the tub or shower. The wall behind a pedestal basin where spray from hand-washing runs. The area around the toilet base. These spaces see water without anyone calling them wet areas. They are, though—they receive moisture, splash, and spray regularly enough that bare substrate will absorb water and fail. A waterproof assembly is not a product applied to a finished surface; it is a layer built into the wall or floor structure, under the visible finish.
Splash and vapour are two different loads. Splash is direct water impact—a specific area that gets spray or splash regularly. Vapour is the ambient moisture in the air after a shower, humidity that condenses on cold surfaces and can soak into permeable materials over time. A waterproof assembly under the entire floor addresses splashing at the perimeter. A moisture barrier in the wall assembly—behind plasterboard, under insulation—addresses vapour load. They are different problems requiring different solutions.
A waterproof assembly is continuous and layered. It sits under the finished material. The finished finish—paint, tile, plaster—is not waterproof. It sheds some water for a time, but water eventually finds its way through and past it. The assembly underneath catches that water and directs it back out or lets it dry. A painted wall with no moisture protection underneath will absorb water through the paint and into the plasterboard. Plasterboard saturated with moisture fails. The assembly is what prevents that failure.
The visible finish is never the thing keeping water out. Paint is not waterproof. Tile and grout shed water but are not waterproof. Plaster is not waterproof. They are weather skins, buying time while the assembly underneath does the actual work. If a waterproof assembly is not built in, and water gets behind the visible finish, it has nowhere to go but into the structure. Understanding this distinction is the foundation for lasting waterproofing.