I was on a budget as well and ended up getting the Rokinon/Samyang 7.5mm fisheye paired with the Nauticam 4.3mm dome port. The lens is only $250 and is even sharper than the 8mm Panasonic lens! I also got a good deal on a second hand dome. The pair ended up costing much less than the price of a new 8mm Panasonic lens alone!
I put a thick rubber band (cut from a motorcycle inner tube) around focus ring and was able to mount the Nauticam zoom gear for the 9-18mm lens. I can now focus manually using the zoom gear and focus peaking. Since this is a fisheye lens, there's a lot of leeway for focus anyway. It's great for close focus-wide angle shots as well.
The pictures come out really sharp. The only disadvantage is not being able to change aperture during the dive. I usually pre-set it to somewhere between f8 and f11 depending on the intensity of the sun and the expected depth of the dive. Manual focus is also slightly slower but not really an issue.
Hope this helps!