I have used the 3M Pinstripe Removal Wheel. Here is a link where you can purchase one:
3M Pinstripe Removal Wheel
I purchased mine from a local industrial supply store. I have used it to remove pinstripes from a 1979 Lincoln Continental, as well as old registration numbers from boats. The wheel works excellent, leaving nothing behind.
One suggestion is to "test" an area of the boat surface that is not easily seen, to make sure the wheel doesn't do any damage. Depending on the size of the decal, it shouldn't take you more than a few minutes to remove.