My rule is 9 p.m.
I do make exceptions, though. I might start an hour or two earlier at parties, cookouts, and campouts. And I always drink starting around noonish if we go canoeing. For some reason it doesn't get to me then like other times, at least till we get back to shore and I get dried off and SOMEONE ELSE drives home.
I like the idea of margaritas in the pool though! I may have to try that, if my pool water ever gets above 75°F. Which it will, 'cause we just put the solar cover on, so there!