As a career drunk, I can say definitely that your responsibility is to yourself. Take care of you. You can do nothing for him unless he wants to quit. Nothing will be gained by going down with the ship. It's possible that your leaving may give him the jolt that causes him to realize he needs help. But don't count on it. He's on crack's schedule, not his own or yours.
Get out of there! Take what you can safely take but get somewhere safe. When you're on dry land you might be in a position to toss him a life preserver, but if you try to go into the water after him you'll both drown.