My bad. That's a good one, it also offers some good resistance. I remember when I spent some time vacationing in the Philippines I'd go running barefoot on the beach with water past my knees. What a workout! Normally I run up a hilly landscape that would take the best of me, but with water, and sand it's a whole different experience. I also read some time back that the sand is good for the Achilles tendon, for your ankles and the arch of your feet.

On another note, both Rocky Marciano and Muhammad Ali would shadowbox a few rounds in the water. One account from one of Rocky's long time friends is that after boxing a few rounds on the heavybag, he'd go jump in the pool and mostly threw left hooks and uppercuts.
I have reason to believe that Rocky's acute handspeed, and prodigious hitting power can be accredited to the fact that a lot of his early effort went into baseball.

I won't go into detail but I remember reading in one of the books on him that he was examined by a doctor after he had a shoulder injury a doctor remarked on his impressive handspeed. After that Rocky couldn't pit a fastball from home to second, but his shoulder wasn't a factor in boxing.