Underwater the Gopro doesn't have more than ~94° squeezed FOV, flat port issue:(. Actually I am looking for a cheaper alternative of the UFL-G140.
4K video is consider a "premium" right now, except for the LX100. I prefer 4K over 1080p mainly because of the flexibility of cropping, down sampling to reduce noise, but not at $5000 price tag with GH4. I don't know what the FOV of the 8mm fisheye lens would be once it's behind the dome. I haven't encounter manta ray, whale shark or anything that size so I don't even know if the 8mm lens would be enough. But I'm quite happy with the Opteka .4x. for general size like turtle and shark.
I recently just acquire the 72mm version Opteka .3x (http://opteka.com/OPTSCFE.aspx). It could work underwater and if that's the case, I should be able to get more wide field of view.
Another option is use the goPro Hero4 as a supplement, so if you need really wide angle (more than 100degree), use the goPro instead. Since it will be close distance, there may not be the need of white balance as long as the ambient light is sufficient, or there is enough video light.