Buffing fiberglass is not too different from buffing a car. You will absolutely need a single-action, multi-speed buffer for this job. If you don't have one, forget it. Your arm will fall off before you're done. No, the random-orbital type will not work. Those are junk. Never buy one.
Remember a few key points:
To begin, I sanded the hull with 220 paper in really bad spots (deep gouges that I could feel with my fingernail). If you can't feel it with your fingernail, move on to 400 or 600 grit paper and a bucket of water. Dunk the sanding block with the 400 (or 600 if your boat isn't too badly faded or chalky - your choice) attached into a bucket of water, sand, rinse, sand and so on. Sand in straight lines, not circles, and sand with the direction of the hull lines. If you do end up with sanding scratches, you want them to follow the hull lines so they're less noticeable.
After wet-sanding with 400, move on to 600 and repeat. If you've already used 600, move on to 1000 and wet-sand everything again. Spend extra time with the 1000, don't get impatient. It needs enough time to remove the scratches from the 600.
Once you're done sanding, wipe the boat down and dry it. You should have a consistently hazy finish that looks like the shine might be there, but is hiding under a consistent haze. Now it's buffing time!
I used Wizard Turbo Cut for this step. Using the single-action buffer and a general-purpose buffing pad (the foam kind), pour a circle of buffing compound onto the pad (use something midrange and professional - go to an auto body shop and ask them for buffing compound that you'd use after wet-sanding). Dab the pad onto the surface you're about to sand to keep the compound from spraying all over the place when you start the wheel. Fire up the buffer at about 1/3 speed and keep it moving. Don't do one area too much, and pay attention to where you're buffing so you don't hook any screws or hard edges. Buff the area until you see only a light haze being left from the compound. I like to buff until the compound is gone. Finally buff it all again with a finish cut compound.
Next check your work. If it's shiny, you're in good shape. Look for sanding scratches, primarily. If you see some, you'll need to experiment with more buffing or wet-sanding to remove them. That'll be your call.
Last step - give your hull a coat or two of carnuba wax. The gelcoat should come out looking like new.
A few remaining tips:
Scratches and even some stains should come out with this process. Regardless, your boat will be shiny and good looking when you're done, and if you keep it covered and waxed, the shine should last a long, long time.