Depends upon the type of adhesive
and decal material, but as stated gentle heat works well for vinyl (heat gun). And lots of different solvents can work for removing the goo, "Goo Gone", WD40 or similar, mineral spirits, carb cleaner spray, paint
thinner....I"ve used all the above at one time or another.
You will have an area of the hull
which is different shade of color than the surrounding area, but that will weather
in over time. Might motivate you to buff out the rest of the hull