Several years ago, I lucked out in finding a deck chair on an upper deck. There were a few empty seats around it at the time. A few minutes after I sat down to read my book, a woman came along with towels, blankets, books, flip-flops, and some other everyday objects that she intended to use as seat-saving devices. She sat in one of them for about a chapter (I lose all track of time when I'm reading on deck), then announced she was going to wake up her family and asked if I could hold her seats.
Now, not only is she doing something that isn't fair to other passengers, this was putting me in a difficult position. So, I asked her how long she was going to be and, while I don't remember the exact amount of time, it was something like a couple of hours. I flat-out told her no -- for one thing, I wasn't sure if I'd be there that long -- I didn't mention how uncomfortable it would have made me. Thankfully, she was nice about it. The last thing I wanted was a confrontation. But, I don't even save seats for my own family. Why do it for a total stranger and a dozen of her friends?
People will try to save seats whether towels are free or not. They'll just find something else. Oddly, and thankfully, this seldom happens on the promenade deck.