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.