i spent about 30 minutes on this until I looked it up. SPOILERS AHEAD
What ended up working for me was, do not use boost. Kill initial boats and head towards the rear of the valkyrie, but not too close. Kill next 2 waves of boats, but kinda stay put, dont get too close to valkyrie. Then a plane will sink the big boat next to the valkyrie. Another 2 waves of boats. Then a chopper appears. Down the chopper, then gun it with boost to the rear of the valkyrie. It will start moving at this point so you have to hurry.
Basically, you have to trigger the event of the plane sinking the other ship, then kill more boats to trigger the helicopter. Once heli is down, make a mad dash for the rear of the boat.
Kinda wish I read this a few weeks ago. Got stuck on that mission and restarting it god knows how many times.
After my first play through though when you get back up top on the ship, when it separates, does it show you kicking the door, then the door being closed. And then you magically on the other side about 1/4 way through the animation of the ship tearing apart? Happens to me ALL of the time now. Gives me sad face :( --- PSN: Shiro1516