Sounds to me like the person asking this question has already found something that will lessen pain and maintain fitness. Again, by consulting a specialist in this area, they can be pointed to exercises specifically for the neck and spine. The buoyancy support of the water decreases the compression stresses on the weight bearing joints and bones and muscles in the legs. Most people can exercise in the water with minimal muscle soreness. The possibility of injury is almost eliminated.
Blood circulation is increased.
The resistance of the water also makes exercising a bit of a challenging type of activity so you can strengthen faster. From the mental aspect, also, it's great for a general feeling of well being. Certification, for healthcare professionals like myself, is generally new. I think there was a need in the industry to set certain standards and the association that I'm certified through, which is the Aquatic Therapy and Rehab Institute, spent literally years determining what those standards should be and how to go about making sure that aquatic professionals meet those standards. There are certain contraindications to aquatic therapy.