I realized I had never gone to the Sea Caves but for some reason I had sold my scuba gear. Without the scuba, I don't know of any way of getting to the Caves. After hours of frustrating travel across various areas, it became clear that none of the merchants had the scuba gear for sale anymore. Finally I gave up and, out of desperation, leapt into the water leading to the Baijin Shallows, thinking that maybe I could just keep one character alive with Heavy Heal Potions until they reached the Sea Caves. Everyone quickly died - except RFS-81! He then bravely carried the corpses of his companions through the Shallows, fending off Dwellers with liberal applications of Death Bombs until we got to the Sea Caves and everyone could get resurrected.
It's sad how happy this whole thing made me....