You can always put a small fleece or flannel blanket in the bottom of the tub or sink too so they won't slip then gently pour the warm water over there back. You can set up a cup with some oatmeal soaking in a sock in one. Sometimes it helps them get used to the water without panicking as much. I do this for foot baths for Sandra in the sink as well except I won't pour it on her back and will just pour over her feet. The blanket on the bottom really helps get the poop off as they walk over it in the water. Just another tip for your arsenal
Edited to add: I think I remember Nancy giving this tip and it really worked out well so I incorporated it with mine.