These sunny seaside tourist destinations have a limited window in which they can pour revenue into matched capacity - solar power output to connect to desalination. They need enough to keep things running when reservoirs are dry, and a plan for either using the electricity when there is enough water, or banking the fresh water when the need isn’t there.
This really needs serious planning and economic modeling. Perhaps they can do solar such that the output matches the AC need during the summer travel peak, and then match that output to desalination for the off season, banking the fresh water in the form of refilling aquifers.
Those are specifics, the broader issue is capital cost and duty cycle. Matching AC and solar is great, because production occurs when the need occurs. If you tie a desalinator that can run 24x7 to purely solar output, you’re only using a half to a third of the machine’s capacity. This was the constant concern with wind driven ammonia production; you have to get creative on what to do when the sun isn’t shining and the wind isn’t blowing.
Another angle to consider here is pumped hydro storage. There’s a lot of steep terrain on this island and not enough fresh water. If it’s possible to build a seawater impoundment up high, excess solar electric could be used to pump seawater uphill during the day, then the outflow at night drives electrical production.
Reverse osmosis requires pressurized salt water at between 40 to 82 bar. Every 33.5 feet of water column adds one bar of pressure. The mechanics here are not clear to me - it could be possible to create a solar plant that fills a seawater impoundment during the day and then match the desalination to either the pressure available OR the 24x7 electrical production possible from reservoir outflow.
The capital and O&M costs that seem nonsensical to us due to our being used to the pre-climate change regime may come to be seen as brilliant by future generations. But that’s going to require acceptance of what we have done, combined with foresight and a tolerance for risk.