Under the thermostat is the intake manifold, yes this drains when you remove both block drains AND the bottom end of the big hose at the front circulation water pump.
How I do it on a Cobra:
warm up engine, change oil & filter
run engine again, fog engine through carb
let engine cool off that water will be HOT.
then, remove the bottom end of the big hose at the circulation pump.
remove both block drains, poke the holes to make sure water comes out
remove the drain plugs in the exhaust manifolds, poke those holes too
remove the raw water intake hose at the thermostat housing, point it down in the bilge to drain
on the outdrive are 3 drains, 2 on the starboard side, one on the port side, you have to tilt the drive up to see all 3, remove all 3 and poke em to see if water comes out. Lower outdrive, that helps them drain. Usually water comes out of only the one on the port side, this drains the raw water intake hose/impeller housing area
after draining, coat threads with gasket sealer and replace the 3 plugs
OK after the engine & manifolds drain....
re-connect bottom end of big hose to circulation pump
put gasket sealer on the drain plugs for the block and exhaust manifolds, and replace the plugs.
for filling with AF:
disconnect the top end of the big hose from the circulation pump at the thermostat housing, then fill it with AF till it comes out the neck of the 'stat housing. Now the engine is filled, just reconnect the hose.
then, hold the raw water intake hose up and fill it with AF till it runs out the water intakes on the lower unit, then re-connect the hose; this pushes any residual water out of the hoses that go through the transom mount and PS cooler mounted back there.
Next disconnect the exhaust manifold feed hoses at the thermostat housing. Fill each one with AF till it runs out the exhaust housing on the bottom of the transom mount (you'll hear it). Then reconnect those hoses.
I use a mix of Sierra no tox AF and water and measure the mix with a refractometer. I don't think the -50 or -60 are good enough for climates that go down to or below zero in winter. For that you need -100.
I've been doing it this way for 20 years, never had a problem. DO NOT take short cuts, this way everything is drained and the inside of the engine and manifolds is protected from corrosion. Many in fresh water regions do not use AF, that's ok too if you are sure you drained it all. I am in the salt, I think the right AF helps.
