Sam as jepski drain the block, manifolds, remove lower water hoses and drain them. Then reconnect them and remove top water pump hose and fill with AF and reconnect. Remove manifold hoses and fill with AF (or until AF drips from exhaust), then reconnect. I leave the AF in for the winter. I don't have knock sensors so I use plumbers tap on my plugs and they look like new and I do not get any rust when draining and my impellers last for years.
We see -20,s many times each winter. 07 boat and never had any issues and my other inboards the same.