simple really, it's someone who keeps an eye out for his friends/mates/buddies etc on a night out down town
12 lads go out down town, while one lad will be choosen to stay sober, his job, is to keep an eye on things and notice anything that could go wrong, to either tell the group to leave and go to the next bar or stop it before it happens.

Don't worry, everyone has there turn, plus side, who ever is on shark watch gets his cokes free all night :-)
by buzzinbeeny April 12, 2008
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