When I vacationed in the UK several years ago, we often saw public parking lots where the parking was free, but limited to an hour or two, depending on the local rules. To enforce this, the parking facility provided little cardboard clocks, with a clock dial that could be turned to show the time that the car was parked, and placed face up on the dash. The rules were that if you tried to cheat and set the clock ahead of the current time, the meter maid would ticket you, and if more than the allotted time had passed, based on the clock setting, you also were ticketed.
So my suggestion is to have the cruise lines have these little cardboard clocks available around the pool. If you need to leave for a few minutes, you set the clock for the current time. When someone sees a chair with clock set for more than an hour previous, the chair is fair game. And if you see a chair with clock set more than 15 minutes in the future, the chair is immediately available.
And heck, if the cruise lines will let me cruise for free, I'll put on a skirt and be the meter maid (although you won't like it)!