Personally, I use aspen in my tubs, along with a plastic (silverware drawer rectangle pan with a 2" hole cut in the top/bottom) hide and a water dish they can also hide in. All my snakes use one or the other to hide under even though the tub itself would seem like a hide itself. I would guess the get the added comfort of touching the sides with their bodies (something I've heard/read numerous times).
D80
PS. But then I'm pretty anal about the comfort and care my snakes receive. You should see the records I keep . . . !