The showerproof meshes are created with 2 seperate meshes, a top mesh and a bottom mesh.
Adult Males:
amtopnaked and the ambottomnaked
Adult Females:
aftopnaked and afbottomnaked
Elder Males:
emtopnaked and the embottomnaked
Elder Females:
eftopnaked and efbottomnaked
Teen Males:
tmtopnaked and the tmbottomnaked
Teen Females:
tftopnaked and tfbottomnaked
With these filenames you create the showerproofmeshes the same way you create normal clothing meshes, only you have 2 Meshes now instead of one.
You also have to extract the cres and shape like with clothing meshes but link those to the skintonefiles.
Here you find how to do so:
http://www.modthesims2.com/showthread.php?t=89397