Wow, we are dealing with a lot more power here than I originally realized. I imagine there would be a ptc availible in that range, but I was thinking of a single cree light when I suggested it.
With a cheaper build and a driver that wasn't adjustable like the shark, the PTC between the led and the driver, the ptc would steal some of the power away from the led and the driver should keep steady output, still putting out say ~1amp but a portion of that would be re-directed into the thermistor, dimming the light (depending on the amount of resistance chosen).
With the shark it is even more ideal because it would act as a variable sense resistor to actually control output from the driver. Since the sense resistor is only seeing a small portion of the output from the driver, it would ideal.
I don't know it for a fact, but I am guessing that is how the magled module works, or probably most any drop in that "throttles" back the output when hot.
Here is one article that discusses the use of them for led drivers:
http://www2.electronicproducts.com/...nt_control-article-farrepcs-jun2007-html.aspx
So, in theory it would work very well. But, in practice, it may be a bit harder. Just finding the right value and rating for one thing, and proper placement of the PTC in your design. Obviously the design is dependant on accurate temp measurements, and I would think as close to the leds as possible.
Now, this isn't to say your thermal switch wouldn't work just as well, there is more than one way to skin a cat after all, and this is your design. Just attempting to offer you some options.
One other slight weakness I see in your design is the MCPCB. It is just one more thermal junction to deal with. A bare emitter with the corners trimmed off should give you a better mechanical connection.
But, I have to say, you are on your way to one heck of a dive light! I can't wait to see it :twothumbs
Actually get an IR thermometer and see what real world temps you're getting, then get a proper thermocouple to match.
