I agree with dvc93cats. You can buy a refillable mug at Goods but it costs (I think about a buck but less than the regular soda) for a refill. It works out, if you are staying near the main pool area.
OKW's refillable mugs have always been discounted, not free. We bought ours in 1992 when we joined, and when we remember to pack them, we still get the discount.