Brad, I think it's great that many of us "baby" our diesel engines as it will certainly help to keep them running less problematically over the years.  That being said, they are diesel farm tractor engines modified for marine use.  My buddy has a similar Kubota block engine on his farm tractor and he does not winterize it or change the filters more than once every several years when he happens to remember to do so.  He runs it in the midst of winter with an oil pan heater and it has been working great since 1988. 

Myself, I change the air, fuel and oil filters annually, check the transmission fluid level/color, check the fuel/water separator filter, check the belt for wear, check the coolant level and color and the zinc in the heat exchanger.  When it starts to run hot I remove the heat exchanger and soak it in muriatic acid and check for blockages.  Always check the water impeller and carry a spare.  This spring I can see that I need a new gasket under the thermostat housing as it's leaking a bit.  

For the coolant I use NAPA diesel premix 50:50, either red/pink or green and drain out the old color if I'm replacing with a different color.  I've never worried about a flush, much less using distilled water.  It's a diesel, a workhorse, not a high octane gas machine.  Doesn't need much babying. I did pull the injectors after 31 years and had them serviced, two were not fully functional. 

Biggest learning I had from the minor overhaul last year was that the rocker arm clearances are SUPER important to check under the rocker arm cover. Not a tough or big job, but if they're out by a hair one or more cylinders might be stuck open or closed leading to over heating or mis-firing.