From everything I've read, water therapy seems like an excellent way to exercise...with these cautions..
make sure the water is cool enough. Many pools are way too hot, and it's easy to get overheated. Some therapists know this and arrange to have classes either in cool pools, or in pools first thing in the morning, before they get heated up!
Make sure the therapist knows about people with MS and what you can and can't do.
Finally --for me, going into a pool would be an exercise in learning how to survive with all that WATER around!!!!! Yikes!!!! But if you're not afraid of the water it should be great!
...I am not a doctor, nor health professional, and don't pretend to be one, here.....