I agree, I prefer to winterize while still in the water too. All around, it's just a lot easier.
That sounds like a good method you used when your boat was hauled out, but I would have thought it unnecessary for a "fresh water cooled" engine (by which I assume we both mean an engine equipped with a heat exchanger). On our boat at least (Yanmar 3GM30F), the thermostat is not in the raw water/heat exchanger loop, but rather is in the closed "fresh water" side of the cooling system which already contains antifreeze. So it's not necessary on our boat to warm the engine up in order to get the winterizing antifreeze fully into the raw water system.
Does your boat have a thermostat in the raw water side of the cooling system? That seems improbable, but who knows? Certainly no harm in doing it the way you described, but I'm wondering was it necessary? Perhaps I've missed something...