Open to anyone. I like the one at the Poly best, simply because the lockers are free to stow your stuff. (It's not much money, it's just a psychological thing.)
Be sure to ask about discounts (AP, DVC, Disney VISA, , you never know what).
Definitely do it at one of the MK area marinas, as you have much more wide-open water, and you can go faster and have more fun.