Well you can use a heat gun to soften the adhesive and then scrape the letters off. Then follow up with 3M decal remover. This method works good. The decal remover will take off the rest of the glue left behind.
Now for the bad news. Depending on how long the letters have been on your boat, there will be shadow from the old letters. The older the decals the more it will show. In some cases you can even feel a bump where the letters were. If you have an old boat it will probably have quite a bit of gelcoat on it. You can try using a buffer with compound to try to get it out. If that isn't working try, wet sanding with 600 grit, and then go back to the buffing compound.
By this time you are a whooped puppy and ready for a beer or some other adult beverage. Now when you go back you might still see a very faint outline of the letters but they will fade with time. Your boat will look great from 10 yards.