To paraphrase Dirty Harry "Do yah feel lucky"
if you don't drain the raw water before sucking in the af, its all a matter of how much the thermostat opens, because using this method THAT is the only way raw water can exit the engine. If you think you can tell when the thermostat opens, it can work. On my engine with the OMC/Volvo style stat housing, the way you can tell is....at first the hoses from the stat housing feeding the manifolds will be cool, because the stat is closed, and all the raw water is going out the exhaust. When they start to get warm, that is a sign that the stat opened, and raw water is now exiting the engine via the intake manifold water passage. BUT, there can still be some raw water in there, you really can't guarantee how much exits one way or the other. Not only that, using the -50 or -60 AF this way is especially risky because it is not that strong of a concentration of PG antifreeze, and more likely to be diluted. Using the -100 which is much more expensive would put the odds more in your favor.
This just brings up the main problem I have with I/O boat manufacturers, and I/O engine marinizers...they don't really care about mechanics or maintenance, all they want to do is make it cheap and pretty and push it out the door. If they cared they would:
1) make the engine accessible, you REALLY don't need those stupid steps that take away 1/3 of the engine compartment space, and seating next to the engine can easily be made removable, it just takes a little more time and thought.
2) these are GM automotive engines, they were designed for a pressurized cooling system with antifreeze in it. All inboards, no matter if I/O or straight inboard, should be closed cooled. That would eliminate all this risky foolishness taking short cuts, etc. Outboards are designed for raw water cooling, inboards are not....they are slightly modified car engines...imagine running your Chevy V8 in a car on raw water and having to run out every time it dipped to freezing to drain it, what a stupid system!
Most people who use these methods don't really understand how raw water cooling works, and if you've never had a problem, well then either you've been extremely lucky or it really does not get that cold where you live....
here we have to prepare for occasional dips to zero degrees....