The easiest way to do this is to fill a 5 gallon pail with non-toxic
anti-freeze, the pink stuff rated for 100 below zero. If the boat is
still in the water, warm up the engine first. With the engine shut
down, disconnect the water intake hose at the strainer.

Agreed, this way you're pulling anti-freeze in the same way as the water is
handled.

Just BE SURE to do it only when the engine has completely warmed up.
Otherwise the thermostat won't be open and parts of the system would still
have water in them.

-Bill Kearney