I've got black and dark red gelcoat on my boat, which is a serious PITA to keep looking good as every little scratch shows and waterspot up (just like swirls and waterspots on black auto paint). Imagine that, waterspotting on a boat. Never would imagine my gelcoat would be getting any water on it.
The Meguiar's Marine polish seems to work OK for getting out the swirls and scratches. My boat has no oxidation, so I can't comment on how well it would do on gel coat that is in bad shape. Folks tell me carnuba is still best for protection, even for boats, so I use Clear Kote Carnuba Wax right now but might switch to a paste when the bottle runs out. I have a bottle of Babe's Waterspot remover and Clear Kote Quick shine on the boat for a wipedown at the end of the day. I find that if I do regular quick wipedowns to restore the shine, the need for major detailing is reduced.
The Babe's water spot remover really works like a charm, even on hard water spots that won't come out with the Meg's Polish and a cutting pad. A few sprays and they wipe right off. Remember that if you do get waterspots and have to get them out with the Babe's (or vinegar solution), you have to re-wax that spot. The Quick Shine is great for keeping the gel coat clean and making sure fresh water spots don't become permanent hard water spots, it also works great as a general multi-purpose cleaner on the glass and chrome parts.
Lastly, if you moor your boat, get a boat lift. If you trailer it, then just keep it covered and wax periodically and it will stay looking good for a long time. Boats that get all nasty and oxidized are seriously neglected. Treat it like you treat your car and it will be fine.