You could upgrade the camera system on deck to include an AI program that automatically notes when an item is placed upon a deck chair, and starts a 30 minute countdown to track whether the item moves or any other additional items are placed on the chair within 30 minutes. It could also track and monitor body heat to identify whether a person is actually using the chair.
When the 30 minutes are up, the AI system activates an onboard drone which swoops down, lifts the item(s) from the deck chair, and then deposits the items (1) in a collection bin; or (2) in the sea [Programmer's choice]. The deck chair is now ready for a new person.
I can envision this system simultaneously tracking all deck chairs.