NEVER immerse a rabbit in water. I learned from personal experience that they can become extremely stressed, enough that they could die, when plunged into a bath. These animals keep their fur clean in much the same way cats do.
Nevertheless, there are situations when bunnies need their hind ends bathed. In my When a Man Loves a Rabbit: Learning and Living with Bunnies memoir, I wrote about a terribly-neglected fuzzy lop named Harry who was given to me by a friend of a friend. This is what I did to make him half-way presentable to the vet.
Rabbits aren't bathed like dogs, needing only an inch or two of warm water to be washed in. Since his paws and butt were the dirtiest part of him, I didn't have to fill the tub much. I made sure the water was tepid and that I had towels handy to dry my bunny off afterwards. Since I didn't have baby shampoo, I had to make do with hand soap. Then I fetched Harry.
I knew that rabbits disliked water but I didn't realize how much of a fight the poor guy would put up. Harry squirmed, wriggled, and tried every trick he knew to get out of the tub. He even attempted to leap out twice but I pushed him back in. Then he stared at me with the most reproachful look I'd ever seen. Harry must have thought that I was torturing him the way he gave me that "shame on you" look. It took a while to soften the faecal material and peal it off of him. The job would have been easier if he had held still. Both of us were glad when the ordeal was finally over.
Harry still smelled a bit but not nearly as strongly as before. I set him on some old towels and dried his fur as best as I could.
I wrote about Harry, as well as a number of rabbits who lived with me, in my When a Man Loves a Rabbit: Learning and Living with Bunnies memoir. Please check the links on the left hand side of this page for information regarding this paperback and Deliverance from Jericho: Six Years in a Blind School.