chiming in with my theories...
Ben is one step ahead of everything and is where he wants to be because he goes back and forth in time.
Ben sent the therapist to find Juliette by time travel
Also, the person on the boat would probably be more of a shocker than Michael, I could be wrong here but I think it would have to be more shocking for Locke since Ben told him to sit down.
I'm thinking it could be 1 of 2 things... if they are playing the time travel game... Ben could say to Locke "my man on the boat is YOU" if they're going to go there. So if they DO go there, all bets are off.
if they DO go THERE, then my vote is that it's Locke's dad, because that'd be a shocker.