Groove it out and weld another piece of plastic in the crack. Then take another piece and build up over the top of it. This wil take some time, but should be stronger than any other type of repair. There is another guy on youtube that uses a special heat gun and is a pro and has a shop where he repairs them. Depending on which NuCanoe you have you might be able to go through a hatch and add some plastic from the inside. If you have a Classic you might want to even cut an access port in the top side. I don't currently have a NuCanoe but really like them and have owned them in the past great boats. You might call the Kayak Store in Hartselle and see if they know someone that repairs them.