The lockers, bathroom, and shower are all located at "Da Waterhole".beach bar on Mullet Bay. You pay for the pass at the bar. Try googling it on Trip Advisor.
There is another bar, Rosie's, but I am not sure if they have bathrooms. A new place, Kalatua, has opened since we were there last year. I am not sure which place you had in mind. We have been going to Mullet Bay back when the bar was a cooler, the grill was a 55 gallon drum and the bathroom was a port-a-potty. Da Waterhouse was a big improvement after the hurricane.