Ok, if you're talking about the mission where you're out in the sea of Moh Koh Phi Phi island I have not finnished that mission either, when I get to last stoppage point, and stop, the mission doesn't end, I've tried this many times and no matter how close I get to the bouy, it won't register, is that what problem you're having?
P.S this really should be in techinical support