My method for my GT40 has worked through some VERY cold winters in Minnesota.
1) Warm engine up, preferably by driving it around the lake, but otherwise back it down the ramp and let it idle until engine temp is 175 and oil pressure drops to 20 PSI at idle.
2) Change oil while warm.
3) Drain ALL plugs (2 block, 2 manifold, 1 water pump U)
4) Remove and dump strainer
5) Remove inlet hose from trans cooler and hook up 5 gallon bucket with hose attached to bottom
6) Put plugs back in with fresh teflon tape for next year
7) Put 4 gallons of the cheapest RV antifreeze you can find in 5 gallon bucket (FVP at Menards is $2.19/gal)
8) Start and idle boat. When bucket is halfway empty dump in last gallon.
9) Let engine suck all antifreeze out of bucket, and stop engine when bucket is empty. It's just coming out the exhaust at this point
10) Reconnect raw water intake hose, put a splash of antifreeze in raw filter housing, and reinstall.
The $11 in antifreeze is worth it for peace of mind. Needed? Maybe not. But if it displaces some water that might freeze and keeps a boat from needing an expensive repair, not to mention screwing up part of your next season, that's good enough for me. We regularly get -30 where I'm at.