Ok, cool. I think the way we could handle this is by having the new PCs come from the crew of the Wyvern or Nixie given we're not at a port. New players could have the choice of a named NPC from the crew list, or as to now unmentioned lower deckers (either way let's say their abilities to now were not accurately estimated when they were recently employed). Or maybe they were stowaways. They can then volunteer (or be volunteered by the master at arms) to join the rescue party.

I think 3 extra would be a solid number.