Re: Yanmar 1GM10 won't start.
Have a read of other threads here relating to 2 GM and 3 GM as they all work fairly much the same way; can all be a bit tricky to start in very cold weather. Work through one issue at a time. Check for something really simple to begin with, like the decompression lever (or a valve) stuck open. That one will be obvious because you will be able to spin it over by hand.
While you are messing around cranking it over, keep the seacock CLOSED because if you haven't yet got water in the cylinder you sure don't want to add that to your woes. Just remember to open it as soon as the engine does fire up.