I just had an idea. The cruise line could zip-tie little 30 minute kitchen timers to every lounger - you know, the mechanical ones where all you do is twist the knob and they start counting down. (For this purpose, though, they should probably be silent, like those hot tub safety control things.)
Anyway, so if you have a lounger, and you need to get up, turn the timer to start a 30 minute countdown. If you are looking for a lounger, and the timer is running, then you know they really were just here and just got up to go re-beverage, or de-beverage, or whatever. And if the timer is off, then you know that person isn't coming back any time soon and you can move their stuff.