What happens if you drive a PBX off the map?

[Stark (DayZ) 0]

Ok, so I got my PBX near Guba unstuck at low tide and drove it to a proper spot, then got out to look around the hill to make sure it was clear. When I come back 2 minutes later, my boat is gone and in the distance I see it being left at the edge of the map with 2 guys there. I run up the hill and over, and by the time I get there the boat has disappeared and the two guys are gone, and the location where I saw it is right on the line where the water stops.

So what happened? I'm befuddled.