I wish you hadn't asked this, and I hate to appear rude in my response, but here goes:
This is a peeve of mine (of many people, in fact) - people who think it is okay to put some personal item on a chair and go off to do other things thinking they have a right to the chair for the day. Reserving chairs is against the rules - on any cruise line. You will not find any employee willing to do it for you no matter how much you try to tip them. A personal hero of mine was the pool attendant on the Mariner Of The Seas who would watch people do this and then he'd saunter over and remove their towel/book/flipflops from the chair.
Don't do it.