Disclaimer: I'm not an attorney, nor do I play on TV...
I'm gonna guess that all other things being copacetic, being intoxicated on a boat at anchor is not illegal and therefore not liable to BUI.
Much will depend on how local LEO's define "operating a vessel" though. Does that mean underway? Engine running? Keys in the switch? I've heard many places will cite you for DUI even if you are parked, with the engine off, but the keys are in the ignition.
But, as others have said if you've done something to draw LEO attention to yourself or your vessel then I think it safe to assume that they will (among other things) be assessing whether or not you are impaired. If said attention is due to boat-on-boat contact, then it won't matter who was at fault -- the impaired boater(s) is/are probably looking at a tough time ahead.
All of the above notwithstanding, I'm also in the group who advise keeping a sharp and sober lookout at all times. Why take unnecessary chances? There are plenty of reasons that could force an unplanned departure from an anchorage. If you really want to have everyone aboard buzzed, find a marina and tie up -- not very likely to get cited for BUI there.
Last edited by PorFin; 09-06-2009 at 10:31 PM.