If Gavin, a race horse slated to race in today's Kentucky Derby can do it, so can YOU! Water running is one of the best cross training exercises you can do if running is contraindicated. Water provides buoyancy, decreasing load on joints while at the same time providing added resistance.
Most injuries that require rest from loading of joints will benefit from water running. Those with ligamentous tears of the knee should not water run until cleared by physical therapist or doctor to do so. Those with incisions post surgery should ask their doctor first before entering a pool.
Ask your PT for specifics on how to water run!