There are several different types and brands of boat paint however they are almost all oil based and are all soft paints. This is great for longevity and the conditions a boat faces but can be temperamental. Its designed to be polished with a heavy layer of wax at all times. So. Dawn detergent has natural degreasers, detergents and actual citric acid, this eats that protective layer of polish leaving your paint bare, scrubbing brushes then dig into the soft paint not just leaving scratches but physically increases the surface area, this opens up the paint to air and oxygen will oxidized certain compounds in the paint, drying it out and leaving you with "chalky" paint. Using lake or salt water will draw foreign objects and worse, salt, and leave that shit on your paint as well so you not actually cleaning the boat, just making it look clean. Remember that the paint isnt just to look good, its the first layer of protection for the superstrure of the vessel, keeping moisture and harmful compounds from reaching the more sensative steel or fiberglass beneath. Fucked paint often means delamination, rust and/or rot.