Should be easily correctable in post. I realize that's not the answer you're looking for, but sometimes we gotta do things we don't really want to do... The D500 holds up well to even heavy-handed exposure adjustments - going from bright to dark is much better on noise than dark to light. Personally, I don't expose to the right anymore. I'd rather underexpose, ensure my subject is sharp with no motion blur, and then bring up the shadows in post. The amount of detail you can bring back from an underexposed photo, to me, is more than you can bring back from an overexposed shot with significant clipped highlights.
Your histogram will look very different depending on the conditions or type of shooting you're doing. The histogram on a blackwater dive will look much, much different than the histogram of a correctly exposed shot in a perfectly clear spring-fed stream. I don't know what conditions you're shooting in, but images with dark(er) backgrounds and strobe-lit foregrounds/subjects should show a bunching up toward the left of the histogram (blue ocean water won't bunch as far toward the shadows as, obviously, jet black conditions at night), but it should gradually taper off into brighter zones. If the taper ends abruptly in the first or second zone then you've underexposed, perhaps too much (though see above for my thoughts on this).
Clear Caribbean Sea water or clear spring-fed creek water under bright sun will probably yield a more classic hump-shaped histogram with brightness values gradually increasing from the shadows to a peak in the midtones and then tapering off toward the highlights.
Kinda hard to explain without using histograms as examples, but at least maybe this will give you an excuse to go for another dive!