I don't remember seeing any lockers at the pools themselves, but there is a locker room/shower/restroom right next to the Pau Hana room on the ground floor of the Ewa tower, which is just a few steps away from one of the lazy river entrances (not far from the Makahiki). Those lockers are free for guests, though I think mostly it was used by people who were checking in early/checking out late.
Other than check in and check out day we just left valuables in the room and took only our key cards down to the pool since you have charging privileges on that anyways.