I thought that you are considering A7 MII and not A77 MII. If that is the case, I would push my preferences very slightly toward Nikon, simply because it is not clear what will happen with the Sony A system. Sony A77 M II is great camera, and while you can expect similar image quality like with Nikon D7200, it has transluscent mirror technology that is taking some light (1/2-2/3rd of EV in my experience). Lens offer is more limited han in case of Nikon. I beilieve that you should find more accessories, such as underwater housing for Nikon and while I didn’t investigate, I think that you might find also some underwater flashes which will communicate with Nikon iTTl.
For all those reasons, Nikon will be better option IMHO.