My suggestion is to leave your towels at home! We brought ours and they were never used. Not to mention that there really isn't room to hang them up to dry when you get back to your room - they would be in the way.
What we did was go to the pool area near the end of the day before a port and grab two towels each. Then we took those towels on our excursions. Just packed them into our bags with our other stuff. When we got back, we put them in the "used towel" bin as we reboarded the ship.
Other people were "checking out" the towels as they got off the ship and the crew was keeping track of who took towels. However, we didn't have to do this - we already had ours and no one said a word.
I will say the the "check out" line for the towels was congested each time we got off. It was nice to by-pass this confusion. Hope this helps.
I believe the cruise lines are quite clear that pool towels are not to be taken off the ship.
Celebrity provides "beach towels" in the cabin so there's no "check-out" line. I think that's a really nice service. You just dump them in the bathroom (or wherever) at the end of the day and they are replaced. If, at the end of the cruise, you don't have the towels in the cabin, your S&S card is charged.
I believe that HAL suggests you ask your cabin attendant for beach towels ... again, no "check-out line."
My personal sin is snagging a beach towel toward the end of the day to wrap up in after showering. Nobody's ever said anything about that, either.
I travel quite often in the Caribbean. Beach towels not only take up a lot of room but are sand magnets and hard to get dry. My friends and I take paraos (sp?) which can be used for bathing suit coverups as well for sitting on the beach. They're easy to shake sand out of and dry fairly quickly.