Hi, towels can be rented ($2 each I think), lockers are a flat rate (small or large) with a returnable $5 deposit. I'm not sure of the exact locker cost...maybe something around 12 or 15???...Please help me out on this friends=)
A good tidbit about locker rentals is once you pay for it once that day, you can keep the receipt and have free locker rentals the rest of the day at any other Disney park too

I think you just pay the $5 deposit & get the $5 back when you return the key...rental cost taken care of with your receipt.
Finally, the good news is parking is free at the Disney Water Parks!!
Have a fun trip!