Shawnmelito on Instructables writes:

THE PROBLEM: When the family goes swimming, either in a pool or at the lake, my daughter, who has Cerebral Palsy, would either just have to float around in a life jacket, or be carried around by an adult. Either way she had no independence. Also, because she is so skinny and isn’t moving around, she would get cold very quickly, even on a warm day.

After a few disclaimers to use your common sense and supervise your child at all times, you can find the general notes on building one of your own water scooter for a disabled child.