A castaway

 is a Sunless Sea Explore Random Event.

Trigger conditions
 can be triggered in Mutton Island and in  Whither by doing the following:

Event description
A woman in an iridescent green gown waves vaguely from a rock near the shore. Sorrowful sighs waft across the water towards you. An idiot castaway? A mermaid? Of course not. Mermaids are a preposterous myth.