There is this blueish / greenish marine epoxy I think you can get at Walmart. Its one of the ones that comes in two tubes and you mix it up. Get some between the top and bottom sheets and down into the foam core where it's exposed. Then duct tape over the top sheet and pull tight towards the tip of the board and wrap it down over the bottom sheet (holding them together while the epoxy cures) Once that's done apply some more of the epoxy to fix the molded fin. Generous amount and then sand to fix the shape.