I seriously doubt that it's even possible for your water heater to light gas vapors from your boat under normal condition. The LEL-UEL for gasoline is about 1.5%-7ish% if I remember correctly. Your garage would have to be heavily saturated with vapors in order to reach that sort of level. Let's say you have a 20'x20' garage with 8' ceilings (technically not even big enough to fit most boats these days into), that would equate to roughly 3200 cubic feet of airspace. You would need approximately 48 cubic feet of concentrated gas vapor just to reach the lower explosive limit in that space, and the space would have to be completely airtight. No garage door on the market seals well enough to allow that kind of concentration to be reached. If there are cases of this actually happening, I would be surprised and extremely skeptical that there wasn't another underlying cause. The only way I could see this possibly happening is if your water heater is in a contained closet and you were dumb enough to put an open gas can in there with it.
Last edited by FastR3DN3K; 01-24-2014 at 6:09 AM.