Before you pull your boat out for winter. You should bring your engine temperatures up to normal operating temperature. Then you drain your oil and remove your filter. By heating up your oil any junk in your lower end will drain out with the oil. Your new oil in the engines will not absorb any condensation over the winter months. What you don't want to do is leave your old oil sitting in your engine over the winter months.
#3 as well. 35 years never had a problem! I start the engines every weekend even if we are going no where. I also 1/2 through the summer, check the oil and pump out about a pint of oil, but replace it with Marvel Mystery oil, run it for a couple of hours and change the oil. So I do it twice a year. Oil is cheap and prevents problems.