Overwater rooms carry the highest rates on most tropical properties, and only part of that is the view. The structure itself is expensive to keep standing in a way a beachfront room is not.
Salt attacks everything continuously
Salt spray and humid marine air corrode metal fastenings, hinges, air-conditioning coils and electrical fittings far faster than inland exposure. There is no dry season for this process.
Materials that would last decades on land are replaced on a cycle of a few years, and the fittings that resist corrosion best are the expensive ones. Maintenance is continuous rather than periodic.
Timber decking sitting above salt water swells, splits and requires regular treatment, and a failed board over water is a safety issue rather than a cosmetic one.
The foundations sit in a moving environment
Piles driven into a lagoon bed carry loads into sediment that shifts with storms and currents. Scour around a pile base is a normal process that has to be monitored and corrected.
Inspection generally requires divers, and repairs below the waterline need marine plant rather than a ladder and a truck. The cost per repair hour is several times a land equivalent.
Storm loading is also different. A wave striking the underside of a deck delivers an upward force that a land structure never experiences.
Access constrains every job
Materials arrive by boat, are carried along a walkway, and cannot be staged near the room. A job that would take a morning on land becomes a logistics exercise.
Work is also usually restricted to hours that do not disturb guests, and often to the closure season, which compresses a year of maintenance into a narrow window.
Environmental rules add process
Building over a reef or seagrass bed is regulated in most jurisdictions where it is allowed at all, with permitting that considers shading of the seabed, wastewater handling and construction disturbance.
Requirements differ widely between countries and change over time, so what a given property may do is a matter for local authorities rather than a general rule.
Compliance work such as monitoring water quality and seabed condition is an ongoing operating cost, not a one-time approval.
Why fewer of them are being built
The combination of permitting difficulty, construction cost over water and lifetime maintenance means many new properties choose beachfront pool villas instead.
That relative scarcity supports the rate. An American traveler comparing an overwater villa with a larger beach villa at the same price is choosing position and novelty rather than space.