Diesel Sheen after running engine for Hot Water

I belong to Windward Sailing Club in Newport Beach Ca. We have 15 sailboats from 28 to 44 feet which gives me a great opportunity to experience difference boat sizes, manufactures and systems. Recently on a 4 day trip to Catalina Island on a Beneteau 36, I ran the diesel engine to get hot hot for showers. After 20 minutes I was surprised to see a diesel sheen around the hull emanating from my exhaust. I immediately shut down the engine to allow the sheen to dissipate and then cut our trip short thinking that there was a problem with the engine running to rich. Has anyone experienced this before? Any solutions I could use next time?