I don't know much about hydrotherapy, but since I had a google hit querying cerebral palsy and water, I thought I would find some information on it.
At UCP there was the following article: http://www.ucp.org/ucp_generaldoc.cfm/1/7044/7045/10697-11782/4189
My favorite paragraph:
After recently visiting both therapeutic swim programs; Special Needs Aquatic Program (SNAP) and San Francisco Children’s Swim, it’s apparent why both of these offerings have full waiting lists. Here, children think only of the fun they are having in the pool. At the same time, they receive so much more: mobility, physical therapy, self confidence and social connections. For some, as you’ll discover in the program descriptions below, this type of workout is a key differentiator in the overall health of our special youngsters.
We tried general swim lessons with D when she was about 12 months old but without much success. I think our main issue was that the pool was a standard pool and simply was not warm enough. She would enjoy it for about five minutes before she would stiffen and just cry. So, we pulled her after the second class. However, at her therapy center, Hollyridge, they have a hydrotherapy pool. She hasn't been prescribed the services, but I think it would be a fun and therapeutic activity for her and most kids with cerebral palsy.