I assume that you are talking about spray on finishes. My understanding is that you can''t substitute automotive for marine finishes. Automotive paints are generally an acrylic base. They are intended to ''lay out'' to a very high sheen and to easily blend to an existing paint edge. They go on as a much thinner (mils) coating. They lack the adhesion, moisture resistance and UV protection characteristics of a marine paint.
Marine paints tend to be linear urethanes. (There are linear urethane modified acrylics used in automotive work as well but they are formulated to look like automotive acrylics and are fully as expensive as marine paints and not as suitable). Marine LU are not as shiny which is important since boat hulls are not as fair as car bodies and automotive high sheen finishes tend to highlight every defect beyond what is acceptable.
There are no real advantages to automotive finishes except a wider availability of colors and the ability to do ''fade in'' type special finishes. The really big costs in terms of painting a boat is in prep work. The fillers and primers are a large proportion of the material cost and of course is the major labor cost as well. Saving a few bucks using an automotive finish is really penny wise but pound foolish. At least that is how I see it.