100% acrilic latex house paint over a porch and deck base coat works great.all good quality house paints come with 15-20year guarantee's.they are used under all climatic codition's.i have used them on wood,steel and fiberglass boats.they work.i always thought the marine enviornment called for only a marine paint.wrong!100% acrilic latex paint is equal to or superior in many ways to marine paints.plus its relatively inexpensive,and easy to work with.
You are right that marine paints are not the only paints that can be used in a marine environment. Most of the problems with paint in marine use is in the preparation of the hull, and the application processes used. I must say however, that in my experience I have never found an acrylic paint that has been wholly acceptable for outside use of any kind, let alone in a marine environment. Bearing in mind the cost and problems involved in correcting any mistake, I prefer to stick to proven processes.