I am an advocate of aqautic exercise for restrengthening injured runners and athletes. I also use it to train my clients that are not injured. Aquatic exercise is used in almost all professional sports programs for prehab, rehab and postrehab situations. And equestrian (horses) training facilities spend big money to construct pools/tanks for use to train multi-million dollar horses, both as training and rehab for injuries. It is even possible to train for power and hypertrophy using aquatic exercise.
As with any type of program, medical clearance is advised.