Surely as long as you pump thro' a fresh water/antifreeze solution you don't need to have adrain plug? This is what I do and have never had a problem - as the waterlock is a purpose made ss one it hasn't got a drain plug.
dickh
I'd rather be sailing... /forums/images/icons/smile.gif
dickh is right . anti freeze solution pump thro should suffice
Frost damage comes in enclosed spaces such as engine blocks. Since the exhaust pipe will only be partly full the frozen exhaust water will tend to squeeze out lengthways before it will have enough lateral pressure to burst a strong reinforced enhaust hose - that my experience anyway. If you dont believe me try an experiment in the freezer and let us know what happens!!
You need sufficient capacity in the silencer/waterlock to allow for churning on the starter motor, which might otherwise flood the system and fill up the cylinders.
Otherwise when winterising, use antifreeze pumped through from water intake, - also is an anticorrosive, and supposed to be good for neoprene impellors.