Here's what I think.
Ok, we know he is drunk as hell. I bet he tried to walk back to his room or something, decided he had to puke, and leaned over the railing, puked so much that he got dizzy, and with the blood rushing to his head, just fell over board.
Cause I'll tell you what, when I puke after drinking, i have no energy to hold my body, sometimes you puke so violently that the toliet seat stuck to your forehead is the only thing that keeps you from drowning.
Those are my thoughts. He puked so hard and so much over the railing while hanging over it, that he just went a little too far.
why he didn't die, is cause he was probably so drunk, that his body just took the impact with no struggle at all. I've heard of drunk drivers being so drunk, that they fly through the windshield of the car and don't ever get a broken bone because they have no resistence to fight the motion and therefore they take the path of least resistence through the car.
We can agree he is lucky as hell to be alive.