Exactly, the thermostat functions as a gate valve, and due to the fact that with an engine running at idle, it won't build enough heat in the cooling water in the block to keep the 'stat fully open, you really can't safely use the suck AF up the outdrive method with raw water cooling. Boats are designed with excess cooling capability so the excess always goes out the elbows, and the 'stat simply opens to let water that's hotter than it's opening rating out of the block.
Now if you have a closed cooling system, either a full or half system (block only) you can use the suck AF up the drive method because it is going to fill the raw water side of the heat exchanger, the raw water pump (impeller) and manifolds & elbows (if a half system) or the just the elbows (if a full system).
So I completely drain mine after fogging the carb and after making sure every drain hole actually drained, including disconnecting the bottom of the big hose on the front circulation pump, here's what I do:
replace plugs in block and exhaust manifolds, I like to put a bit of Permatex Aviation on the threads to prevent corrosion
re-connect the bottom end of the big hose for the circulation pump
disconnect the top end of that hose at the top by the 'stat housing
put a funnel in it and fill the engine with AF till it spurts out the 'stat housing neck, then re-connect the hose, now the engine is full of AF
disconnect the feed hoses for each manifold at the 'stat housing and fill each one till the AF runs out of the exhaust reliefs under the transom mount (you'll hear it draining on the ground)
reconnect those hoses, now the manifolds are full
Disconnect the hoses for the impeller (mark them first if a Volvo)
(it's a bit different with the Volvo than my OMC which has the impeller in the outdrive)
Take the hose that goes back to the transom mount, hold it down to drain, then with the outdrive down, fill that hose till the AF runs out the outdrive water intakes. This pushes water out of the hose , the PS cooler and passages in the outdrive.
With the Volvo style impeller I'd mark the hoses first and do as I just mentioned, but also, reconnect the hose that comes from the transom to the impeller, and disconnect the hose from the impeller to the 'stat housing, I would fill that hose with AF too, that way your impeller housing is full of AF and won't have a dry start in the spring.
I think this way you are 100% covered for winterizing. You can see why outboards have taken over the pleasure boat market. People got sick and tired of this rigamarole every fall. And you forget once or wait too long once and its new engine time!
