If the towels are on the pool deck, you most likey will have to sign them out, using your sign/sail card and be sure to return them. If you don't return them, you will get charged for the price of the towel. Also, feel free to take them with if you have a shore excursion that you may need towels.
Pool attendants will check them out to you. You can also get them for shore excursions to the beach, etc. The charge, if you lose one, is $20, but well worth the risk not to (1) use valuable luggage space (2) trying to get the sand and water out of a heavy beach towel (3) packing nasty sand covered beach towel for the trip home.
Funny you mention beach towels. Last cruise there was a family of five and the father was freaking out about a $100 add on to their sea pass at the guest relations desk on the way off. Turns out they thought they could bring the towels home.... seriously?!
Good one!! How could you be so dense as to think you just get to keep the towels? I think he was freankin' out cause he got caught (). 5 Towels??? Who has extra room in their suitcase for 5 towels? Wonder what other items made their way into his luggage that he thought they were allowed to just keep?