I use an annual plan from DiveAssure. It includes both primary medical as well as dive insurance, and trip cancellation/interruption coverage. You choose the amount of coverage you want to have when you buy the plan, and that amount will apply to each trip regardless of the cost of each one. I don't know if it covers other extreme sports. It's also not cancel for any reason (and I'm not aware of any that are); covered reasons only, including weather, illness, etc.

Also, Dive Assure plans cover ONLY international trips. I know you said you're doing international, but just be aware that they won't cover domestic (USA) only.