If it's the vinyl with adhesive........time and patience. LOL, just kidding, but no seriously. There are a variety of adhesive/sticker removers and debonders out there. I've always been leary that they will cause discoloration of the gelcoat. Obviously if you're going to use one, test it in an inconspicuous place first. I've had good luck with a razor blade. The sturdy kind that go into a utility knife, not the rectangular kind that are used in box cutters. You have to hold the blade at a very, very low angle, just barely enough degrees from the plane of the boat to hold onto it. Avoid any lateral motion as that causes a slicing effect. Lateral motion or too steep of an angle will cut into the gelcoat. Too deep and it will be tough, if not impossible to completely buff out. When done, I use a 3M Color and Gelcoat restorer. This has a bit more of an aggressive cutting agent than Cleaner and Wax, yet a bit less than a rubbing compound. It also doesn't have any wax in it. This buffs out any marks made by the removal process and blends the line at the edge of the graphic. It also removes any residual adhesive. You may find you'll need to use the gelcoat restorer on the entire boat because the decal covered perfect gelcoat, while the rest of the boat has seen 10 seasons of sunshine.
Painting the boat will be very tricky. There are brands out there like awlgrip and others. These are typically 2 part paints, with one part being the 'paint', while the other part being a hardener. These need to be mixed in a specific ratio, then applied in small batches because the mixture hardens as you work. It requires a lot of finess. For the best application and finish, it typically needs to be mixed with a thinner and sprayed. Rolling and tipping, or brushing and tipping are very difficult and can end up with mixed results (Not always). It can be done, but it's certainly not like painting the bathroom. For what you're describing, I'd be considering consulting a professional. It's probably not as expensive as you think. Especially if you do most of the prep work yourself.