I know that this thread is old, but next time anyone encounters bad gelcoat, find some 3m Super Duty compound. 95% of the worlds worst gelcoat wont stand a chance against Super Duty and a wool pad on a rotary. Forget all the tips and tricks, just give the boat a good wash to make sure you don't contaminate your pad with dirt/salt/anything else that may be on the surface. If the gelcoat isn't bad enough to the point that it's chalky, try a less aggressive product. After the compound, finish with 3m finesse-it or something similar before a wax/sealant. The 3m Imperial products and the Finesse-it can be used on painted boats too.
Also, you shouldn't even consider wasting time with a DA on any gelcoat, especially if it's chalky. Having said all that, you better be sure the surface you are about to hit with the Super Duty/Rotary is in fact gelcoat and not a painted surface. I've seen numerous boats that have been refitted, where the hull may be gelcoat and the flybridge is painted. Super Duty can get you into trouble but there's no replacement for it either.
Boats are really not as bad some guys on this forum make them out to be. I'll gladly spend a week taking my time on a 70ft sportfish over messing with clear coat depths, finicky paint types and products non stop for 2 days on an exotic car. Detailing a car is more of an art, whereas boats are just basic knowledge and brute force in a sense.
Here's a link to purchase 3mSD
Shop 3M: Super Duty Rubbing Compound
I haven't just detailed a few boats, I've spent the last 11 years working on 70-250ft superyachts as my day job. I may start working on a good boat detail writeup but I haven't had time recently, I'll eventually get around to it.