Take a regular old fashioned 12v electric fuel pump with a couple of lengths of pipe attached. Moor up beside the target, pop the inlet pipe into their fuel tank, the outlet into your own, switch on and sit back. No siphoning, no noise, no commotion and a steadily emptying tank.
No I'm not recommending it. But a few years back, in spite of police attention a couple of reprobates were thieving their way around the Kent river moorings draining off hundreds of gallons of fuel without attracting particular attention. On one unfortunate occasion the police were even watching a mooring where a boat was drained, while it was being done! They just thought it was normal mooring activity. Apparently the two oiks even came up and lounged on the cabin roof of their boat with beer whilst their pump pumped! The police were so unimpressed they didn't even note any descriptions. It was some months later, after the offenders boat was seen going under the Kings Ferry Bridge with a suspicious number of Jerry cans in the cockpit, that they were rumbled.
Alan.