In an area that flexes from weight and next to a scupper there might not be a good repair. True plastic welding is the best repair for cracks but is difficult and tool is some what expensive. Plastic melting with an iron of some sort are not that good and long term. Hard epoxies will crack off from the weight in the seat and flex of the hull. You might try some Lexel. It is better than silicone and stays flexible but won't stop a crack from spreading. Other than that we need some pictures to see where and how bad your crack is.