I have customized several kayaks with decorative vinyl stickers designed for autos from Hippy Motors USA and custom lettering from Rapid Vinyl (they sell on Ebay). Very happy with the products from both companies in terms of price, ease of application and durability. Hippy Motors has some fun stuff including “steampunk” and traditional Maori designs and they are a small US based family business that cuts them to order in your choice of colors. Rapid Vinyl’s order process allows you to format your text exactly as you want it with over a hundred scalable fonts.
The stickers adhere very well to rotomold boats (see photos of boats that have had the stickers on them for several years: Sanskrit “om”, peace dove, boat names). I have even reimforced the text banners to the fabric skins of my folding kayaks by covering the lettering with strips of clear nylon adhesive repair patch material (bought at REI). Some people scoff at decorating a boat in such a way, but, as I learned in my years of relying on a bicycle for transport, thieves are less inclined to take a customized item both because it is too easily identified and because the resale value is less when they try to “fence” it.
By the way, the vinyl stickers can be removed by heating with a hot hair dryer if you want to change designs or sell the boat.