As far as the physical part of removing decals, a heat gun is the best way but it takes some practice. Don't just heat a spot, heat a large section of the decal. A plastic scraper is the best thing to use to lift one edge, then pull and heat, keeping a large section (a foot or so if the decal is that big) warm.
Done correctly, the decal will come off with the glue intact. Too cool and the decal will tear or the glue will remain on the gelcoat.
Acetone and paper towels are good for removing any remaining glue.
As for removing the shadow or "ghost" left by the decal, the answer is, you can't. The ghost is the area of gelcoat that has been protected from weathering by the decal. The surrounding area wasn't protected. You can't make the area under the decal look like the rest of the boat, the only thing you can do is try your best to make the rest of the boat look like the area that was protected by the decal.
So, start with an agressive "fiberglass restore" product. If that doesn't work, move to rubbing compound. If that doesn't work, wet sanding is all that is left.
You may be able to blend the affected area in or you may have to do an entire section of the boat (like the transom).