Complex sentence: Poop-Decker, Sweetly and Sally thus help the Captain into a boat, and they leave the ship, but while leaving his cabin, the Captain gets a splinter in his foot, which later goes gangrenous.
Split into simpler sentences: Poop-Decker, Sweetly and Sally thus help the Captain into a boat, and they leave the ship. Leaving his cabin, the Captain gets a splinter in his foot, which later goes gangrenous.