Oil changes should be done annually (more often if you run a lot of hours) even if you only put a few hours of run time a season on your boat. This is because of the lake environment that we all love can be hard on your engine. Moisture is one of the many by-products of combustion and when you combine that with the moisture laden environment it shortens the life span of the oil.
We use ONLY marine grade oils and premium filters for our oil changes. Cost per engine runs $80 labor plus parts; parts may very depending on if it is a small block, big block, gasoline or diesel.