The fish eye conversion lens is interesting. Are you referring to the Panasonic DMW-GFC1? If that's the one, then I don't think it's wide enough because it only give 120 degree on land, and would become 84 degree once in the water. May be a wet wide angle lens is a better solution, just don't know which wet lens work.
I think Kevster is using the combo behind a 3.5" dome port.