Body surfing works every muscle in your body. Walking through waist-deep water, diving under the waves, maintaining your position and then catching waves makes for a great workout.
Ocean swimming is highly aerobic, and it's low-impact. Plus, it's great for your back because your stomach muscles are doing so much of the work
As well as building upper- and lower-body strength, surfing builds stamina and is great cardio. It works almost every muscle in your body. If you ride 20 waves, you've done 20 pushups.
Running on sand is physically more demanding than running on a hard surface, but less stressful because of the greater shock absorption.