pretty sure it was inland cave, so not sure why the water should not be drinkable and how is it surprising they mention the caves where flooded 60000 years ago

pretty sure it was inland cave, so not sure why the water should not be drinkable and how is it surprising they mention the caves where flooded 60000 years ago, which is long time ago to replace all the original sea water with fresh water from other sources or maybe it's condensated water from walls of cave creating layer on top, though not sure how would that work