Getting past metal dog at the start of the game

The plane has just crashed on a beach and the objective is telling me to "swim to the airplane tunnel". Once I leave the cockpit I get attacked by a metal dog and I can't kill him. I don't see any escaoe here. What am I meant to do to kill it?

Nevermind, you need to Sprint then crouch so that you slide under the dog. No idea how he fits since BJ is a tank but that's what you need to do.