My goodness, what a lively thread! I can see everyone cares deeply about this accomplished missing young man.
I tend to favor Det. Klimpt's theory. I see a bunch of liquored up young guys all ripping up the town. Maybe something happened or was revealed in beer talk that embarrassed our missing subject. Perhaps under normal circumstances he could have dealt with this head on,and effectively. But, perhaps he is one of these folks who "can't hold their liquor" and becomes psychologically fragile when they are inebriated. Sounds like it.
Similar to Det. Klimpt, I *theorize* based on what I have read, that something was indeed done or said to horribly upset/embarass Paul. (And I agree that the roomies aren't giving that info up, or maybe just to LE). I think Paul was so drunk that he completely lost all judgement and committed self harm. I have seen young men act in such a manner inebriated, but usually it's over a girl. If I had to guess, this had to do with something embarrassing related to something relationship oriented, but that's just guessing on my part. There's only so many things that upset a young man that much, and many times it's a relationship.
But in the end, I think a very distraught, and very drunk young man stumbled along outside and ended up falling in, or jumping in the water. That's the best I can deduce from what I know of this case. I think the roomies may be holding back specific conversational details to protect Paul's privacy, vs. protect themselves. I don't think they are guilty of harming him physically. Emotionally, probably, but not physically. I get the sense this is about SHAME and embarrassment.