To be honest I think you'd just make him mad. And after the flare bounced off his chest and landed in the boat, you'd have another problem...
It's a good idea to try out your prospective weapon on something like phone books first, to get an idea of what it will or won't do. You might be disappointed.
Cases in point. I once fired a .357 magnum into a piece of 4x4 scrap from a distance of maybe 10', expecting dramatic results. WRONG! The bullet simply went into the board and stopped. It didn't blow the board to splinters, it didn't even come out the other side. For that matter it didn't even knock the 4x4 off it's perch.
Another time I fired a .45 cal black powder derringer (the type that killed Lincoln) into a piece of 3/4" plywood from maybe 6'. The bullet didn't go through the plywood. It didn't even make a good dent. What it DID do was bounce back and hit ME square in the chest. Stung a bit and left a bruise. That ended my shooting at boards...
We've all been watching too much Hollywood...
On the "Judge": http://www.gunsandammo.com/2011/07/26/410-revolvers/
good god,the question should be, how did this go from being a houseboat forum to 4 pages on what is the best weapon to kill an intruder.
Oh, I wouldn't want to kill anybody. It's just like the Quaker farmer said to the burglar. "Friend, I would not harm thee for the world, but thee is standing where I am about to shoot."