I've removed a few decals from my composite canoes by carefully lifting a corner with a razor blade and slowly peeling away.  It helps to let it sit in the sun for a bit to heat it up before starting.  If the decal tears, I just use the razor blade to lift the edge and start again.  In my experience, most of the adhesive has stayed with the decal.  Whats left I removed using a citrus product - I think it is called Goo Gone.