You check out the towels on the pool deck and then they are yours for whatever you want to do with them. Take them ashore, hide them, make your own towel animals, have your photo taken with the towels, etc.
Seriously, you check out towels and take them ashore with you. It is what they expect you to do. Towels are not available ashore. You could theoretically check a towel out on day 1 and not return it until the last day and they won't care. They have your cruise card info and will simply charge you for the towel if it isn't returned by debarkation day.