Based on when you got out of the boat to repair, I'm almost positive he was able to hop in because you were still in the spawn area for the boat on A.
Based on his history I'll drop a punish but keep this in mind. Thanks!
you can spawn into any boat no matter where it's at on the map as long as someone's in the boat. use this feature all the time as a sort of mobile spawn point for my team. most people are decent when they spawn in on seat one and see me repairing in seat four and move. then other time you get a douchebag.