You can shower and change at the Ship Shape Health Club which is near Stormalong Bay. I'm not sure about lockers. We usually put our stuff in a duffle bag and kept it with us at the pool.
We had a second floor room on the far wing nearest Epcot and it was really easy to go down the stairs and to the quiet pool. Another time we had a standard view room on the ground floor right across from the door that leads out to the quiet pool. Both were very nice. Another time we were on the second or third floor near the elevators and we didn't like that location as well. As you can see, I can't even remember the exact floor/location.
Have fun! BC is an awesome resort.
Donna