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.
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey