That is a common mistake and yes with heavy use it may cause issues. Really most of us on this tread are not heavy use boats. If the engine ran daily (work boat or charter , ect) and with high idle time. It can cause problems over time. As it was said earlier the modern multi weight oils of today are far superior to the oils made long ago. Many yards and mechanics might need education regarding the proper oil. Until I read this forum years a go we used 15w40. Everyone in our area did as well. Drain the oil and filter it will be fine. No reason to flip on the guy, unless you told him do something specific and he disregarded you.