I've never coated the boat, but have a lot of experience with the cars. I would imagine that, like with vehicles, prep is mandatory. Lots of fake ceramics out there today, and most of the dealer ones are, well, a tad weak, and tremendously overpriced as well. The real coatings generally come in small bottles, and require a bit of experience to use.
https://angelwaxwestcoast.com/produc...coating-500ml/. Is one that works very well.
These coatings assume you've taken the time to really polish your gelcoat and free it of all contaminants before starting. I typically polished every spring, and used Gel Coat labs for polishing and protection.
I would never pay a dealer to coat anything. If you have a bonafide detailer in the area, take it to them.