Granted, if you are primarily shooting shallow with good light on a sunny day then low light capabilities aren't as important. I primarily shoot in caves, wrecks below 200 FSW, and at night. In these types of environments low light comes into play, especially if you are shooting in caves with goethite. Cave shoots require a lot of silhouettes on a wide 16-35. I have experimented with several profiles on the Sony A7SII, and I am extremely impressed with Slog3 in these type of conditions. I haven't experienced any banding. It does takes extra time in post to grade but I am happy with the results. I don't recommend Slog3 for every application, I merely suggested to the OP to look into it, since he had switched his focus to a DSLR and was doing research. Of course white balance is important. As far as Sony A7SII, I am very satisfied with the results underwater, and it is a low light monster. I have shot on numerous camera rigs, and have yet to find one that is perfect and without limitations. If you do please inform me.
Why is low light ability so important for underwater video? I can count on one hand the number of times I've had to push the ISO to 3200 underwater. And pretty much every camera from the past few years can handle ISO 1600 just fine. But you'll be shooting in the ISO 100-800 range most of the time anyway. I think you'll be hard-pressed to find many people that agree with you that low light capabilities are more important than good white balance/color reproduction. There's a whole thread discussing the limitations of Sony mirrorless cameras underwater in the video forum.
Advising people to shoot in Slog3 underwater is also quite questionable, especially on a camera that records in an 8-bit h.264 codec. Log encoding doesn't give you more control of color in post. It preserves more highlight/shadow detail. In actuality, you have less color information to work with and banding become an issue much more quickly when working with 8-bit encoded log.
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