Hi Justin. Good question. This posting refers to the Sigma Fisheye/Metabones combination in some detail.
http://wetpixel.com/forums/index.php?showtopic=56631
Included in the first link is Brian Smith's positive report on the combination above water with the most recent software update:
http://briansmith.com/sony-a7rii-canon-ef-smart-adapter-tests/
The other adapter that might be worth looking at is the new Deo Saker Falcon Lite (also marketed as Techart). Not so much information around and only available in UK on eBay:
http://www.deoinfinity.com/#!saker-falcon/cqse
Of course for many types of underwater fisheye use, autofocus isn't really necessary, but you would probably need a custom focus gear for MF. This is what we have done quite successfully for the A6000 with a Tokina 10-17 fisheye and mini-dome (also with a custom zoom gear). I don't think the Tokina would be a good solution for Full Frame.
What about the combination of Sony 28mm FE mount plus Nauticam WWL-1? Seems to get good reports and a lot more compact than dome solutions - although not fisheye I believe with a diagonal field of view of 130 degrees rather than 180 degrees? Phil Rudin reports his favourable experience with this combination compared with the Sony fisheye converter here:
http://wetpixel.com/forums/index.php?showtopic=56866
Mark