Definitely invest in gel coat specific products or pass on it. Meguiars and Collinite have great product lines specifically formulated for boat/RV use.
Be prepared to spend a lot longer than you think. I'd charge $15 a foot if the boat is in decent shape and needs no compounding to $25 per foot if it is in bad shape. The vinyl can be a pain to clean because of the exposure to sea/lake air, sweat and suntan lotion and every single cushion you lift up will have a compartment underneath to clean out. The carpet is usually not to bad to clean. The gel coat can be tough, especially if it is oxidized or has a lot of hard water spots.
You also have to find out how far down the sides of the boat the owner wants polished and sealed because if it is on the trailer, getting the bottom is going to be tough. I'd find out exactly what he wants done and see the boat in person before you decide whether to take the job or not.