As @JonSlayer notes, refraction is a big issue for stitching (because light behaves differently in water, any difference in light on either side of the camera will cause issues during stitching).
You can see an example of refraction causing lines / blurs in this video taken on a GoPro:
Simply put, using the MAX underwater is not really advisable for 360 capture. For single lens capture it works better, but that doesn’t solve your 360 issue.
This issue affects all 360 cameras (Ricoh, Garmin, Kandao, etc.) and is not unique to the GoPro’s.
I second @JonSlayer’s 360Bubble recommendation for the GoPro Max/Fusion (disclosure: Jon is the founder of 360Bubble) – this is the go-to accessory for shooting underwater with consumer level 360 cams right now.
I’ve written up a bit more about how we use the 360Bubble at Trek View here, that you might find helpful.