Teleconvertors are really designed for use with longer lenses, not wide angles or fisheyes; so the way we use them for underwater is not a generally intended use. I don't own a 1.4 Nikon tc or 10.5 anymore, but I believe with the arrangement of the lens elements etc, the Nikon TC may actually not even fit with the 10.5. The Kenko TC on the other hand, will fir, and will work.
Thanks for the explanation! I know what will be in my shoppingbasket next time i virtually shop
The 10.5 is such a fine piece of glass but find the angle slightly too wide for the way I want to use it (underwater portrets)