I have to say I do have a lot of very expensive bottles of marine products “especially made” to be used on boats. We all know that if you see the word marine on a product you can be sure of paying way over the odds for it. But we still buy them.
This article popped up recently in my google feed and makes interesting reading :- Homemade boat cleaners. There is a link in that article for cleaning the boat below the waterline ( please bear in mind they are in warmer climes);-
4 Kidneys and a Wonky Leg 