First of all, car products are for cars and marine boat products are for boats. Not a good idea to use a car product on the gelcoat of a boat. Think about it this way, why would a detailing product manufacturer spend their time and money developing a product for specific use, only to have the consumer go out there and use it for something it was not intended for? Why not trust the product to do what it was designed to do?
When a product is designed and engineered to be used on cars, then use it on cars. On the flip side, stick with using those marine grade products that were designed and engineered to be used on boats. Anyone who recommends using a product for something other than its intended use, will sooner or later learn the importance of this message.
For the exposed exterior fiberglass gelcoats of most boats, I would recommend using a sealer and not a wax. Personally, I have found the Meguiar's Flagship Premium Polymer Sealant to work very well in most situations. Compared to their #56 Pure Wax, the sealant goes on relatively easy and comes off just as easy. And all the while it provides maximum durability. Of course, my recommendations are based on my own experience detailing boats and getting the necessary results.