I'm sharing this in the hopes that it might save someone's life.For those of you who don't know me, I consider myself to be a very knowledgeable and very careful sailor. We just returned from a sometimes challenging but overall very nice month-long cruise on the West Coast of Vancouver Island. But this incident arose from my carelessness, and it could have cost me my boat and the lives of my wife and me.Despite it's being mid summer, morning temperatures at the beginning of our cruise were in the upper 50's in the cabin and I was running the diesel heater to take the chill off until the sun warmed things up. After a couple of days of this I started the heater. The blower and fuel pump kicked in, but the heater failed to fire. After a couple of tries I went to the swim platform locker where the heater is mounted to see if anything obvious was wrong.I didn't find anything wrong with the heater. What I did find was that the plastic gas can used to store fuel for my outboard had become dislodged and was leaning against the exhaust hose for the heater. The heat had melted the can to the point where there was a pinhole in the plastic through which small amounts of both fumes and gas could have escaped. Any further melting would have allowed fumes and gas to be in contact with the heater with tragic results.I went weak in the knees at the thought of the possible outcome. I basically never use the outboard as I row everywhere, and the plastic container didn't get the attention it should have.Lessons to be learned: 1. Obviously don't store your gas can near any heat sources. 2. Even when you think you have the sailing and cruising drill down pat, you can never be too diligent.May we all learn from this.BTW the heater is now working again. Why it didn't fire that morning I'll never know. I think the sailing gods must have been watching over us.Gary WyngardenS/V Wanderlust h37.5