I would recommend bringing a beach towel. The towels at the pool are the small white ones, the same as in the bathroom. Me and Dh ended up buying beach towels in the gift shop (expensive, but very nice). I just like having a big towel.
Moderates have smallish white towels (actually, they're the same towels as at AKL) at the main pools. While I like to have big beach towels at the water parks, I prefer the convenience of the resort towels at the hotel pools.