The lines for lockers are not usually long when we are there. Any cash register in the stores can rent keys...people tend to bunch up at the first one they see, so sometimes peeking a little further will save you some waiting.
We also tend to go first thing when the park opens and then have our fun until lunch time and then split to the Earl of Sandwich at DD for lunch. By that time the water park is getting crowded with long lines and we don't want to be there anyway.
Probably not as much an issue in September.
We get there, get a locker, stuff everything in the locker except for a bag with some towels and some water and snacks. A different bag is in the locker with shampoo, clothes to change into later, etc., after a shower when we are leaving. If the park has just opened, we then head off to our favorite ride, parking our towel/snacks bag someplace handy. After we're tired of doing our favorite ride over and over, we find a centrally located permanent place to leave the towels and stuff and head off to do more rides. We come back for water and snacks a few times.
One thing to remember about the water park is to keep hydrated...it sounds crazy but with all the exertion in the Florida heat you need to keep drinking water even though you're wallowing in the stuff!
Once we've had enough for the day (lazy river is usually our last wind-down ride) we pack up for the showers, get dressed, and head for lunch and a break before tackling another park.