I can tell you this from experience, and it was a very expensive lesson to learn.
When you trailer your boat, you've got your outdrive or outboard(s) tilted up with the tilt locks in place... right? And many of us simply leave everything tilted up until the next time we put our boat in the water. As a result of being tilted up, there is the potential for some amount of water to remain in your engine's cooling water spaces. If you live in an area where the outside temperature never gets below freezing, then you're probably OK leaving things like this.
BUT... if you happen to keep your boat in a place where the temp's can drop, then you really need to tilt your engines back down once you park it for storage. Many years ago, I forgot and left my engines tilted up, and we had a hard freeze for over a week. During this period of time, 'something' happened inside and the vapor recovery chamber on BOTH engines cracked. Apparently there is a cooling jacket around this 2 foot long cylinder and there was enough water left in it to freeze, expand, and crack the outer jacket. I didn't realize I had a problem until early spring when I smelled raw gas coming from my thru hub exhaust. That was about a $800 lesson I learned.
Freezing water can do nasty things. So tilt your motors or lower units down to insure any water that might be hiding in the inner workings can drain out. Also a good idea to make sure your hull drain plug is out too, to drain any rain water or melting snow from the hull. But don't forget to put it back in before you launch the boat next time.