To Remove Decals & Old Registration Numbers

One of the best ways is to use a hair dryer set on a fairly high heat.  This softens the adhesive on the decal and usually lets you get it off quite easily.  (If the decals are fairly old, they may not come off in one piece, you might have to "work" at it a bit.) 

Some will recommend that you use a heat gun; personally I would be very careful with a heat gun to ensure that you don't burn anything around the area.  Plus, if you apply too much heat to the decal you'll find that it will shrivel up and you will be taking it off in pieces.  With a hair dryer, most of the time you'll find that you just want to soften up the adhesive and then you'll peel it off fairly easily.

Also, if you park the boat (or vehicle) in the hot sun, you'll most likely find that this will have the same effect as the hair dryer and you'll peel the decals right off.

If you're really lucky, you'll find that the adhesive will come off with the decal, if you're not and it stays on the boat, try using WD40 and terry cloth. (I've found that terry cloth or something similar will work quite well, paper towels don't work near as well.)

 I've used WD40 many times over the years and it works very well.  (Some will recommend using nail polisher remover, paint remover, etc., personally I wouldn't use anything near that strong on my bassboat ;-)
I hope this helps...........
                                                                                                                        BBC