Ship spawn restrictions?

This has been a question of mine for a bit. I was waiting at a outpost once for an afk friend and a ship seemed to pop up when I wasn't looking towards the dock, less than thirty feet away from our ship which was close to the island, and had the appearance of just spawning due to its lack of cosmetics, and it's positioning to the dock. A friend and I assumed it spawned when no one was looking. That or someone sailed up without us noticing. I'm not complaining about it, but I would like to know if anyone has hard facts on whether or not the ship could have spawned with us still at the island? Or are there physical limitations to how close you can spawn to another ship?