Shutter speeds at certain depths

Well, if you are shallow in sunlight, you should have enough light for 1/160th at ISO 100 and F8 or so, which could give some great hammerhead pics.

If the light is low, you're going to have to adjust your expectations.

If you would like to see how some of my pics came out, and the EXIF data should be intact on the large images (not thumbnails), just peek at the website in my signature.  You can go back to 2015 and see my Roatan pics with an RX100 II and dual strobes, or earlier dive trips where I was using one or another Canon point-n-shoot with on-camera flash (through a diffuser).   Pics from 2016 on were taken with a D810 or a D850, except for a couple of dives in Jamaica in 2018.  (I had to shoot a beach wedding between dives and had no time to deal with getting camera out and back into the housing in time.  So I used my backup RX100 rig that days.  The difference in lens quality is pretty clear to me there, comparing the RX100 to the D810 shots.

But when the light really gets low, you just end up with lack of range, lack of color and noise, as in this highly-processed shot with the D850:

Shot at ISO 100, pulled down highlights 100% and raised shadows 100% in Lightroom.

And here is an ISO 800 shot, taken just after the above as I swam out from the wall and pointed back for an image:

ISO 800, and post-processed

Sometimes there just isn't any chance at decent color, but remember that black and white can sometimes work well in a high-iso shot.

This shot was taken at ISO 400, which didn't seem so high until I tried to post-process it.  Had I not been shooting in the sun I probably would have had a decent chance, but there was so much dynamic range involved that I couldn't capture it at ISO 400 on the D850.   It looked horrible in color, but OK in B&W:

ISO 400

Now some examples from point-n-shoots:

RX100 II

ISO 160, F4, 1/125   Dual YS-D1 strobes.

ISO 160, F4, 1/125th - lighting from strobe OK for subject, but ambient in background is all greenish. Dual YS-D1 strobes here.

Canon Powershot 120:

ISO 250, F1.8, ISO 400, no flash

ISO 125, F1.8, 1/500th   My late wife took this of me.  I am holding the RX100 with dual strobes.

Canon s95:

no flash, ISO 100, F2, 1/60

ISO 80, F2, 1/250  Big difference with later cameras on this sort of well-lit subject.

Canon SD 870is:

For a couple of years my late wife and I both had one of these.   Here is her shooting me:

ISO 80, F2.8, 1/200

ISO 80, F2, 1/320

Canon SD630:

This may be the type of shot you're looking for.   I believe this was quite deep (80+ feet).  At the back wall of Molokini crater in Hawaii.  My wife near a passing Manta Ray.

ISO unknown, F2.8,  1/250th

If you see a red X, it's because DPReview seems to have trouble sometimes caching images from my (homemade) web server.   Just view original JPGs if needed.