for me, renting towels is soooo worth it, use em and dump em, no washing, no drying. We do it every time. Usually get 2 each as thy're small, but then we take in our own drinks and food to save money that way instead.
If i'm staying at a Disney resort, I will just go out to the pool and grab whatever towels I need for the day.
When I am done, I drop them off at the towel drop off's at the water park. Some people say you should bring them back to the resort, but all towels go to a central laundry facility at Disney World anyway... so whether you bring them back to the hotel or not doesn't matter.
or can you exchange it for a dry one?