Since the crack is not in a position to be scraped by rocks I would put some duct tape on it and forget about it. If you heat up the boat and the duct tape just a little bit with a hairdryer it will stay on a good long time.
The roofing tape or window frame tape type of stuff ("bitchithane" or whatever people call it) is stickier and basicallly lasts forever. I use that stuff when I crack a creeker on the bottom, but duct tape is fine for this job.
I tried to patch a crack with ptex once and melted a big ol' hole in my boat. I'm afraid to use anything but duct tape or roofing tape ...they work fine. Welding plastic is way to scary for me.