From Cheddar.

Running underwater, also known as aqua jogging, replicates the exact motion of running while removing nearly all joint impact. The water provides 12 to 14 percent more resistance than air, which means your muscles are working harder on every single stride, burning up to 40% more calories than a comparable land run, with zero pounding on your knees, ankles, or hips.

Studies show that runners who switch to aqua jogging exclusively for up to six weeks maintain their 5K times, their VO2 max, and their lactate threshold with almost no measurable drop in performance. It strengthens the areas runners neglect most, which are hip flexors, shoulders, core, and lower back, simply because water pushes back from every direction at once. Cardiologists recommend it to patients because the heart actually pumps more blood per beat in deep water than it does on land.