Cheap underwater flash

Hi All
I own a Canon Ixus 960 IS with the fitting case WP-DC19. I search now for an inexpensive flash which I could use. One I saw is the Ikelite AF35 which might be useable.
It has to survive diving depths of 40 meters, should use rechargeable batteries and, most important, should work as a Slave which gets triggered by the built in flash.
Any recommendations?
Thanks