We have used the rental towels at Aquatica, as a supply comes with the cabana rentals (I think we get 8 towels as a component of the cabana reservation). They are white and smaller than a regular beach towel. If you rent them at the park, I believe the price is $4 each.
I personally wouldn't pay to rent them unless they came as a cabana inclusion. We would bring our own beach towels and just leave them on our beach chairs. The rental towels get left in the cabana so we can keep track of them.....if I rented towels without a cabana, I'd probably keep them in my locker as it would be nearly impossible to determine who's white towel was whose. There are a lot of the white towels scattered throughout the park.