You can use the towels from the resorts, but they ask you to please bring them back to the hotel. They probably are the exact same towels you rent at the water park, but when they are sent to laundry - parks & resorts only get back as many as they give. The towel will belong in the water park circulation and not be returned to the resort. The resort - in turn - will have to pay to replace the towel... and revenue will have to made up somewhere, raising room rates, food costs, etc...
You can rent towels at the water park for $1 or bring your own.
I've always brought my own, you can usually buy a good sized beach towel for $5.00