My first thought is water in the fuel. Try some fuel antifreeze and see if that helps. Either that or completely drain the fuel system, refill with fresh gas straight from a pump, and add a little antifreeze as well. That's where I would start.
@reogem, makes perfect sense.Hey...grizzlyfish..welcome back.
If you Griz starts fine when in a warm environment then won't start when in a cold environment. Most likely needs the valves adjusted.
Just another thought, and probably @reogem or @dezz or some of the other guys hopefully will chime in on this. It's pretty obvious that if it's sitting out in the middle of a lake it's not connected to a battery maintainer. @JEMacEwan, when yours was outside in the cold was it on a battery maintainer? The reason I ask is this. Mine sits inside an unheated garage all the time. It's always connected to a maintainer. It has not had the valves adjusted yet. It always starts, no problems. We had two weeks of -2 to -10 recently, no problems. My elderly neighbor has a riding tractor with a snowblower on it. I take care of his snow removal for him. Last winter I was having all sorts of trouble starting the thing. It would crank over pretty well, not fire up, then the battery would pooch. Got him a Battery Tender, fires right up every time. I realize the engine is far different, but on that tractor, in real cold the battery just did not have enough juice to crank the engine fast enough long enough for it to start. Suppose this could be part of you guys problems?