I expected that sanding the old paint off the boat's topsides was going to be a pain. It took more than a week. I used 60-grit 6-inch sandpaper on a Bosch random orbital sander with a soft backing pad, attached to a shop vacuum. The paint was in bad shape; chalky, cracking and flaking off. Along the way I saw some repair patches where the boat had taken some bumps. I guess that the paint had been patched there too. The gel coat under the paint was still in acceptable shape - it was crazed but not cracked or falling apart, so thankfully I didn't need to sand off the gel coat too.
Since the waterline was going to be sanded away, I put some notches on the hull to mark the location of the top and bottom of the boot stripe for future reference, to paint it back with its original shape and location. Laser marks can be used to strike a straight boot stripe but boot stripes are not supposed to be perfectly straight; they're supposed to have some shear (widen, and rise up at the bow and under the counter-stern) to prevent the boat from appearing as if it is "hogging". There is no easy way to do that, except by eye-balling it, so using notches to keep track of the location of the boot stripe and waterline is the best option.