Hobie sells a welding tool/kit: 72097001 KC WELDER PRO
It's basically a soldering iron with a hollow tip that you can shove poly "rod" through. It's on page 31 of this catalog:
http://www.hobiecat.com/media/pdf/eKayak_2010Spring.pdfA really quick search seems to indicate that it goes foor 60 to 80 US$. I don't know how much skill it requires. For that reason, I'd go with the $25 solution: fill it with West Sytems Gflex epoxy:
http://www.westsystem.com/ss/g-flex-epoxyYou'd want the "thickened" kit (comes in two tubes rather than squeeze bottles). I've only used it once, but it seems to work. The trick is in the surface prep: wipe the area with alcohol and then wave a propane torch over it for a second or three (not enough to melt or toast anything - just to change surface chemistry). Abrading with sandpaper doesn't help. Flame treatment is key to getting glue to stick to poly, but even then it's an iffy prospect. Gflex is designed for poly and, just as important, is "toughened" so that it flexes like poly (most epoxies are brittle). I got mine at West Marine. If it was me I'd tried to apply it inside the hull if possible - it'd look better.
If you need proof it works, watch this wild video:
http://www.westsystem.com/ss/g-flex-torture-demo/