Anyone have any experience in repairing plastic on Dagger boats, particularly cracks in the back deck around the cockpit rim on Dagger Outburst.
I've had great luck welding Dagger plastic.
It sounds like those cracks aren't in a high flex area (how did they even happen), and that some simple heat gun work might fix them up. Try cutting a small bevel around each crack, then pre-heating with a heatgun or torch, then twisting a strip of plastic into the groove.
If it is a place that flexes a lot, like the bottom of the hull, I've been welding a strip of plastic in -- plenty of tips on how to do this online -- then welding a big flat patch cannibalized from another boat over the top of it all. Chamfer the edges and you're all set.