depending on the decal, you may be able to just lift it off using a sharp knife - just work the blade a bit under the corners and slowly lift and pull - very slowly.  I've removed decals this way several times (like those stupid dog prints on my Bells).  If you are careful enough, the glue stays with the decal and not on the boat, but any glue residue will probably come off using alchohol or some solvent.

Of course, the decal isn't usable again, though some have been almost useable.