Coastal Express (kind like a water-bus) runs as late as 2 am depending on what night of the week.
Private water-taxis run late too - the operator himself will have to tell you.
Both methods of boat transport will pick you up at the Mata Chica dock and drop you in San Pedro in the heart of the night-life action.
Not sure the cost of Coastal Express these days, but Mata Chica's staff can tell you for sure.