Can't really help, but I'm currently looking at one for seeing what all the fuss is about so been seeing whats available (cheaply)
Kodak SP360 4K - Has underwater housing - but suffers from really bad sitiching underwater.
Ricoh Theta V - Just released, has decided underwater (30m) housing and from the example video, it appears there is no visible stitching. I'm off diving next week and if the housing was in-stock anywhere, I would probably of picked one up already.
Garim Virb360 - Waterproof (10m?) but suffers from awful sitiching, but has amazing stabilisation and data overlay functions.
Today (28th) I believe GoPro will be launching their Hero6 and Fusion cameras officially - and if they announce an underwater housing for the Fusion, that could be the camera to get, due to their proprietary sitiching
Camera Rig: Nauticam Lumix GH5/GH5s, 14-42mm II, WWL-1, Atomos Inferno, 48,000lm of artificial sun.