Take Nick's advice, scuff sand the whole boat with 400 grit dry sandpaper, have the graphics done and clear the whole boat.

Marine paint (I am assuming that yours is a polyurethane) has a very short period, when it is first applied, that graphics paint will adhere to it. Usually about 12 hours.Even then the adhesion is questionable.

By sanding you will solve the adhesion problem by substituting mechanical adhesion for chemical adhesion. Just sand the paint enough to take the shine off of it. You could even use a Scotchbrite pad (basically a scouring pad for dishes and graded by colour.)

When you choose your clear make sure it is compatible with the graphics paint and the marine paint. Use a urethane or polyurethane clear.

Hope this helps, Ian