I know on the boats with the older 2 stroke diesels they have heaters that you have to use or they won't start. Block heater or air box heaters. They smoke like crazy when they start.
With the new electric 4 strokes I don't know what they use, never had to start one when it was 80 degrees out.
I know the little Yanmars don't have glow plugs or anything, they just have big starters. You crank the heck out of them and they start.
But on a boat in the water the engine compartment usualy stays warmer remmeber, 30's is cold it doesn't get down much colder than that because of the water. A 617 would probably start in the 30's with no glow plugs.