It's not necessary to shut water off every time. I just loosen the clamp and pull off the riser feed hose and point it in the bilge if the engine cranks too long for some reason. Haven't had to do that since re-ring job. Without glow plugs, when it gets cold, the Yanmars are likely to crank for a while before catching. I use a QUICK shot of ether if it's really cold rather than crank for too long. Releasing the compression relief levers can also increase speed and compression, getting cylinders firing one at a time. Wish these had glow plugs like the little Kubota diesels which start very easily when cold. I think it is a purposeful oversight in design. These engines are certainly expensive enough to warrant this kind of basic equipment but I guess they have little competition. You can bet if Cummins, GM, or Cat started producing small diesels for sailboats, you'd quickly see some improvements.
I have the Yanmar 3gmd, direct sea water cooled. It has lasted 30 years and still going strong. I like the simplicity of the direct sea water cooled engine and if built right, they seem to last. Looks like no one makes a 20 hp direct sea water cooled engine now, or do you know of any?