I take the pool deck as being the whole area up to the doors, since there were patio tables and lounge chairs almost all the way to the entrance. Also, some planters.
But reading Petitfore’s link certainly has me thinking the building was undermined. The leaking from above the garage didn’t help. but a frequent foot or more of seawater in the garage definitely sound like a big deal.
Saltwater should be corrosive enough to an underlying “karst limestone” (as noted by one of our commentators) and any sand above it, to eat away at them to the point that it became a big underground sinkhole without it ever reaching the surface.