Changing your boats oil is a simple yet needed process you should do every year. Just like the lower unit oil, water can get into your oil, though water in the oil is usually a mark of bigger issues. If you have water in your engines oil it gets a milky color to it. If this happens you will have to inspect your seals and for cracks in your oil pan. Like a car, it is best to vhange your oil every season or every 100 engine hours.