When they say "no doctors on board", they mean it. You are pretty much on your own, or at the mercy of the skills of Captain and crew. The wife and I spent a couple decades messing around with voyaging sailboats in another life. Had a friend who was a commercial Captain for freighters, based in Alaska. Captain, 1st Mate, a dozen or so crew. Once a crewmember had a fall, hit his head, and his brain started swelling. Without immediate action, he's dead in 6 hours. Called the home office via SSB radio (this was at the dawn of satellite phones). Got a surgeon on the mic, and he proceeded to direct the Captain OVER THE RADIO how to drill a hole in the guys head using a power drill (Craftsman, I believe...

) and a 1/16" drill bit. The crewman lived, and was eventually airlifted off by a Coast Guard Helo when closer to land.
Now THERES a story to tell your grandkids; "See this scar here, just behind my ear?"