it would probably be benneficial, my only concern, and I am just pulling this off the top of my head, is what the water temp in okinawa is? is it still considered tropical? if so, you would be fine, but if it is cold water, than you would probably recieve reduced benefit.

I would have them collect from a natural beach, i.e. not one where they ship in sand, and I would think low tide waist deep would be plenty deep.