I admire Royal Purple. I am in a similar business and I have met the owners. They have very good products and have done some very good marketing. I personally think they are over-priced but I also think they are the best you can get.
I Use Roy Purp. In My Truck . On Start Up The Engines Rpm Rose About 300rpm. Because It Was So Much Slicker Than Conv. Oil. But Then Computer Lowered Rpm Back To Where It Sould Be. Meaning Less Fuel To Run Engine At Same Rpm. I Did Not Know They Had A Marine Oil. You Can Bet I'm Going To Switch.
RP has a good product but there's nothing magic about theirs compared to other synthetics. The only complaint i've personally heard was from a truck company that tried it for a while. Did a "heavy haul" out of New York and was short 8 qt. at the end of the day - no leaks or usage problems before. Wouldn't think twice about using a named brand synthetic in my car or boat if I had a 4 cycle engine.