To save coral reefs they have to cut emissions of CO2, which has a direct effect on ocean acidification, even without temperature and sea-level rise. The symbiosis between algae and corals depends fundamentally on thermodynamics of carbonate chemistry, I doubt there’s any techno-fix to get around that, however much money they get from their exclusive hotels.
So, Saudis, stop exporting oil.
Planting mangroves does sound like a good idea, but I don’t know if it’d work in that location.