I did a search for shark repellant and bang stick came up. I don't think I like the idea of having something that could blow a hole in my kayak if it accidentally went off. I don't really want to kill or damage the shark. Possibly in my budget though. Here is a link to that http://www.bluewaterhunter.com/shops...angsticks.html

Also found this http://www.sharkshield.com/ they have Jim Sammons on there site. I wonder if it really works. A bit pricy @ 500 for something I may never have to use.

Having encountered large sharks a number of times on a 6' surfboard, I feel pretty safe in a 13' kayak.
The thing you have going for you is that plastic is not a natural food item for any shark I know of.
Take a video of it and be a YouTube sensation.