Shower proof means, when a sim takes a shower he stays in the same body shape like when he's dressed. You need meshes for that.
If you don't have the meshes, he'll revert to the Maxis body shape, or you'll only have a head when he takes a shower.
So how could I do this? O_O The tutorial only helps with reserving the shower proof skin tones. Is there a thread here that explains it?