I beleive you are correct about the voltage on the cattle prod, all the trials were done with 20kv or up to 100kv. A small internal combustion engine is the best source of HVDC for this purpose, especially if you need penetration of the electricity.

I don't have personal experience with marine envenomation, I am still waiting to hear back from a friend about it. I will let you know when I find out.

Macgyver.