Just picked this up. It's wild, and more Lost than I had gone in expecting. This probably would work well as a prequel to Pirate Borg, as a kind of funnel to run before piratey goodness. (Victory in Castaway is getting picked up by a ship, and there's the chance that it's pirates. Using this as a prequel would probably mean it's always pirates.)
A character that survives Castaway would probably be a pretty robust Pirate Borg character, with access to a powerful magic item or two, and probably some mental or physical health issues.
There are new monsters that would work with Pirate Borg, although they tend to be less over the top horror and more serious horror stuff. There's a cool giant hermit crab that lives inside shipwrecked hulls, though, that is just plain great for nautical adventures.