One of the best cleaner/protectants out on the market for your yak is called 303 Aerospace Protectant. You don't have to get the whole kit, you can just get the Protectant.
After a long day in the ocean, it's best to run your vehicle and your yak to a power washer car wash. When you get home, open the hatches and plugs and rinse the inside out and let dry.
Usually after 3 or 4 yak trips it's good to use this product. And if you want to keep the scrapes and scratches down you can use surf board wax on the bottom, messy but does help to slide in dirt and sand.
303 Boat Care Kit
Total boat care made easy!
The 303 Boat Kit is a maintenance kit for all marine surfaces including fiberglass, plastics, carpet, metal, vinyl, and rubber. The 303 Boat Kit includes manufacturer recommended products plus 303 Aerospace Protectant, the only protectant with an SPF of 40. Give your boat the very best preventative care with the 303 Boat Kit!
The 303 Boat Kit includes: