I've been really getting into the 360 this year, and have 2 rigs..
First rig is 7 x Hero4 Silvers, in a 5 sided cube, with top and bottom cams, using the gopro's original housing. Awesome stitches above water, no problem synchronizing cams with Autopano Pro Video 2.5, and Autopano Giga for control points and masking, but I had horrible issues with parallax and stitching underwater footage shot with standard housings that don't have the "domes". Granted mine do not have the dome lenses, and I do believe that is key to the issue with parallax and stitching underwater. Would love the Abyss but it is not in the budget for the foreseeable future. And finding the rig that you use with the domes is hit or miss as it seems 360 heros is pushing the Abyss V4 and dropped your model now.
Footage *was* stitchable, however I had to use Autopano video, and use a template from a previously known good stitch (Saved in Autopano Giga). And that footage had horrible parallax, so was it really "stitchable"? I wouldn't consider the footage usable..
Second rig is a pair of Kodak SP360 4K cameras. I like this rig as workflow is much less (only 2 cams to sync, download, etc), and that also makes it quicker to render. They have a dive housing for the pair which I will be trying out in Utila next month, but preliminary tests in my koi pond show the parallax issues are much less likely to happen. I also like the fact that I can mount the Kodak cams to my drone and get "floating" 360 above the water, in the sky, and make the drone disappear. The 7 GoPro's would be too heavy for the drone. And I did a lot of 360 footage (on land) with the pair of Kodaks recently for a training conference I went to and editing them has been a breeze. I think this will become my go-to rig for 360 footage if all goes well next month in Utila.
Software is KEY! Autopano Pro Video 2.5 for sync, stitch and blend, Autopano Giga for control points, blending, and tweaks, Skybox Mettle so you can do titling and logos without distortion, or even so you can view in 360 format within Premier Pro CC 2016. Yes, software is a tough nut to bust, but man, it makes it so much easier.
Exciting and frustrating at times because there's not much info on 360° video underwater, so its a lot of trial and error... I first tried it with a 3D printed 7 camera holder and SJCam SJ4000's. Thought they would work, and found out the FOV wasn't enough and they lacked overlap. Lesson learned.
Will post vids after my trip of how the Kodak SP360 4K do underwater, on land and in the sky as I plan on doing a passion project / video on plastics in the ocean while I am there, utilizing air, land and underwater shots to tell my story, all in 360..
SO glad to finally find someone else doing underwater 360!!!