Winterization info should be in the Mercruiser Service manuals. Here also is a quick and simple how to but locating drains and such on each engine and year can be different.
http://www.mercstuff.com/winterizing.htm
Also, if you have a sink or head (any type of plumbing) then you will also need to drain and winterize that as well. That is more like an RV though so just drain it all, put some pink antifreeze in the holding tank and pump it through all the plumbing.
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