Frank,
I'm no expert, but I can tell you that my 30+ year old, 16,000 btu, Cruisair (with new compressor in 2013) will cool my H37c throughout the heat of summer in Jacksonville Fl without any extra sun shades.
So it sounds like your AC is working, just not performing good enough to keep up with the heat. Nobody will be able to tell you if it is better to fix an old AC or replace it. If it's over 20 years old, the standard would be to replace it. But as I said, I replaced the compressor in my AC and kept the air handler because it was the easiest thing to do.....but maybe not the smartest.
You asked for help, so here are some basic troubleshooting ideas you may try. Have you had the unit checked by an AC tech to make sure the system is correctly pressurized? Or even before that, have you checked that the cooling coils/air filter are clean? Do you have enough water flow through the heat exchanger? Have you tried cleaning the heat exchanger coils (there are posts in this forum that discuss circulating acid through the system to clean calcium buildup inside the tubing).
If you get no results from troubleshooting, then find a marine AC technician. Ask your friends if they can recommend a good tech. If you find a good one that you can trust, then you may as well follow his advice instead of mine.
Finally, new systems are more compact than the old ones. You may end up with a new empty locker if you install a new replacement unit.
Good luck.
