If the boat is going to sit unused for the winter it's a good idea to change the oil prior to this so the engines sit with fresh oil in them. But if like us you use the boat periodically through the winter then there is no need to change the oil until the engines come due for a regular oil change.
If our boat sits unused for more than four weeks or so without our having a chance or the weather to take it out for a weekend we'll run the engines in gear in the slip at about 1200rpm for 30 minutes or so. This gets them up into their normal operating temperature range for awhile.
