I have been teaching water aerobics for many years, and many participants have benefited greatly from the aquatic environment.
There is a difference between water walking and water aerobics, though. The latter is usually in a class format, often to music, and while often very enjoyable, may be too much for an entry level. In water walking, you would typically do different walking patterns (backwards, forward, sideways and such). It is usually self-paced and outside of classes. Some physical therapists specialize in aquatic therapy, and they should be able to give you specific recommendations.