The buoyancy of the water relieves weight on the joints. Swimming is not a weight bearing exercise, and does little for bone strength. It is an excellent aerobic activity and does burn fat. Water exercises may include adaptations of land activities like jogging, bench-stepping, circuit and interval training, and calisthenics. You can increases resistance to a movement in water by using fins or kickboards.
Safety is an important factor in water exercising.
Don't swim in strange or unsupervised water locations. If you enjoy water activities, it is a good idea to learn CPR and basic water rescue procedures. Find public swimming and water exercise programs that have certified water safety instructors to lead their classes. If you can't swim, enroll in only shallow-water classes. Swimming pools should be clean and safe.