Two fraternity brothers decide to go sailing one afternoon and become lost. After twenty hours with nothing to eat or drink, one of them spots a lamp floating by. He picks it up and a genie pops out. The genie notices the poor condition of the brothers and grants them one wish between the two of them.
After a lot of arguing over who gets the wish, one of them blurts out, “I wish the ocean was made of beer.”
Magically, the ocean turns to beer.
Infuriated, the other guy yells, “You idiot! Now we have to piss in the boat!”

Barry said he learned this joke when he was in a fraternity at the University of Southern California.  He was a member of Kappa Alpha Order.  He told me that one of his fellow fraternity brothers taught him this joke circa age twenty one.  Barry explained that the background for the joke is that fraternity members are notorious for drinking large amounts of alcohol, specifically beer, at a time.  The joke pokes fun at the fraternity life, suggesting that all fraternity members hold their alcohol (beer in this case) in higher regard than hygiene and sanitation.  The fraternity brothers would rather urinate in their boat where they have to live for days on end than in the beer they are floating in.

I believe the meaning of this Barry gave to me is completely correct.  Fraternities are known for binge drinking and this joke capitalizes on that idea.  Also, some might argue that the mild profanity used in this joke (“piss”) could be used to show fraternity members’ tendencies to use profanity at will.