When you get the decals off there is a good chance that you will see the outline of them on the bare hull. After all those years, the hull oxidizes and changes color but the parts under the decals don't.
If the decals you want to remove are the thicker type, try to peel a little edge up and then blow the hot air under the lifted part as you try to pull the decal off. In other words, you want to blow the air between the hull and the underside of the decal. For thin decals you can apply the heat directly to the outside of the vinyl. Keep the heat gun moving and take care not to 'burn' the paint/gelcoat..