These motors are susceptible to issues with frost / ice when laid up over winter. With the Mercruiser there are five drain plugs, one on either side of the block, one on the water pump and two under the manifolds. You need to move side panels to get to the latter.
It is imperative that you talk with your authorised dealer about the wintering process. Your insurance does not cover maintenance issues or preventable events so you should check it out.