Find My falsely indicates my position as North Atlantic

I can't answer that question. It could be from its last confirmed location for all we know. The simple fact is seeing a phones real location depends on it being able to communicate with cell towers. Plus, the phone may not be using roaming services to boot. For example, my brother and sister in law, went on a Greek Cruise. My Sister in law paid for roaming. But only on her phone. I was able to see where she was, but I was not able to see where he was. It still showed his last location in the US, which was in Miami, FL. That's because his phone was not communicating with any cell towers during the trip.

Location Services on iPhone uses a combination of GPS, Wi-Fi and Bluetooth to pinpoint where you are.

Likewise, devices using Location Services also contribute and crowdsource nearby Wi-Fi networks and Bluetooth signals.

It may have been that the boat has a Wi-Fi router onboard that was contributed to Location Services while the boat was in the North Atlantic recently and the devices of the people you follow aren't using GPS and solely trying to figure out where they are based on nearby Wi-Fi networks only.

To prevent this from happening in the future, the owner of the Wi-Fi network can add "_nomap" to the end of their Wi-Fi network's SSID and it will no longer be contributed or relied on for pinpointing a device's location.

More information about Location Services: About privacy and Location Services in iOS, iPadOS, and watchOS - Apple Support

Pinpointing people at sea is very difficult if they don't have a clear connection to either cellular or Wifi when you're looking at their location. This wouldn't be surprising, to be honest.