With my Islander,I turn on the pump and run all the water out of the tank that way,I have 4 outlets so it takes a short amount of time,then I put RV antifreeze into the tank along with some water and run it through the system until all the outlets are showing pink,thats the color of the antifreeze,at that point you should be good to go
I'm not sure which boat your trying to winterize but if you have a water heater you have to drain and by-pass it to pass antifreeze through the hot water lines.