I would suggest you contact underwater camera dealers like Reef Photo, Backscatter and discuss the pros and cons of going from a DSLR to mirrorless. My experience and others that I have dove with is that mirrorless tend to hunt and do not focus lock quickly like a DSLR. So for subjects like moving pelagic fish getting the shot is difficult due to the focus hunting, for stationary macro subjects the hunting is not as bad. Mirrorless advantage is that they are smaller and easier to travel with.
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