Some people will launch the boat with full fuel and let it sit in calm water for a few days to get a visible scum line.
Pull the boat and mark 1" above the scum line with painters tape.
Once you are pleased with your line, prepare the bottom for paint by sanding and solvent washing.
Remember that every last spec of mold release agent must be removed before paint, so if you are not able to do that, having your dealer prep and paint might be a better solution.
The dealer should know exactly where the paint line should be, but discuss it with them first to be certain.
Once it's done, it's done.