One slight problem with your solution
, it's never going to happen!
The cruise lines never even had a taste for keeping jeans out of the dining rooms when they had dress codes, for fear of upsetting on customer. Instead they got rid of the problem by lowering the dress code standards. AVOIDANCE.
So they certainly going to hire extra staff (AT EXTRA COSTS) to police the pool decks, move people's things, and have security threaten customers with violation notices.
Do you also give people a maximum time they are allowed to occupy a prime location, before they have to move on?
The problem isn't the cruise lines, it's the passengers,
who continue to ignore all the printed suggestions for proper and cooperative behavior onboard.
On your next cruise, just stand by the pool and tell your fellow passengers they can't save chairs, and remove their belongings, and see what reactions you get. Do that and they'll need to hire extra security to break up all the fist fights.
Reality is cruise ship pools are small, and for some reason the numbers of people who want the few hundred loungers that surround the pool, is large. A few hundred pool-side loungers for 2500 or 3500 passengers just doesn't work.