I would agree that for underwater, APS-C is plenty good enough. FF might be slightly better for wide-angle, but APS-C definitely better for macro. Most of the advantages of Full-Frame pertain to topside photographers shooting weddings and portraits with big fast lenses wide-open in low light, which does not apply to underwater.
I am shooting the 7DMII and love it. The AF system is awesome, and I think is a bit better than the AF system on the 80D. The 7DMII is only slightly more expensive than the 80D, and has better weather-proofing also. The 80D has features useful for topside like tilt/swivel screen and touchscreen that can't be used underwater. So I think the 7DMII might be a slightly better choice for underwater, but both are very good.