Re: Boat totally in flames . . .
I can tell you one way this kind of thing can happen. I had my own SHT. I had pulled the engine, and when I replaced it, I left the positive battery cable hanging too loose over the flywheel. It wound up touching the top of the flywheel. After a couple of hours of running, there was no insulation left on the cable. I was running the boat and I opened the hatch to check the idle speed adjustment and saw a shower of sparks flying out.
If I had trailered the boat like that, the positive cable would have been bouncing up and down and shorting to ground all the way home. That, combined with fumes from the carb, could easily have caused a similar fire.