Shutter speeds at certain depths

Hmm.

I've shot with a number of fairly limited cameras before getting into DSLR's underwater.   Once I started to take control, particularly with external flash, I started seeing what shutter speeds worked and what didn't.

What I found was that trying to shoot in aperture priority was allowing my shutter speeds to go far too slow, and I got a lot of blurry fins in particular.   Or blurry anything else that moved.   Even with flash this was happening a lot because there was still ambient lighting contributing to the scene.

Bottom line, 1/60th is far too slow unless you have nothing moving.   On my first dive trip with an RX100 II and dual strobes, I ended up changing to shutter priority after a day of getting blurry shots, and I set the speed to 1/160th.   For the past four years I've been wary of going slower as I've gotten many slightly-blurred images when I've done so.   I try for 1/200th or 1/250th with my Nikon DSLR's, and I'm limited by maximum sync speed and strobe performance to those numbers.

When shooting wide angle with ambient, I just cannot use my preferred settings (ISO fixed to 64) - I have to raise ISO, but I can get most shots just fine at an ISO of 400 and shutter speeds of 1/125th to 1/250 and an aperture of F8.

These days I shoot everything in Manual mode, partly because I don't have a TTL flash trigger anyway.

Shutter speed is 1/160th to 1/250th

Aperture is whatever I need for depth of field, which on my 16-35 behind a big dome port is around F8 or F7.1.   For macro (105mm) I start at F16, but that gives too shallow DOF when I'm close to a subject, and depending on the subject I'll kick that up to F29 at times.

ISO is fixed for macro (64 for most dynamic range, lit by strobes)

ISO is ... I'm still in a learning curve here for wide angle - either manual, or a limited range of auto-ISO.  I find ISO 200 fairly nice, until it isn't enough, but I see a lot of loss of dynamic range.   The last trip I took I tried setting Auto-ISO from 64 to 800, and of course I had a lot of shots at 800.  I also had a lot of underexposed shots at ISO 800, and here is where I learned some limits.  I routinely do a lot of post-processing where I reduce highlights and boost shadows to fit a wide dynamic range into a scene.   With underexposed shots at ISO 800, I had no dynamic range left to make anything look good.   Consider that the scene is already artificially boosted (from ISO 64 to 800), so the shadows are already pushed what, 4 stops?    Similar reduction in headroom as I end up with bright subject areas I cannot 'fix' in post.

How you deal with your shots in post will have a lot to do with what upper ISO limit you can stomach.

However, in almost no case am I lowering shutter speed below 1/160th.  I'd rather have a noisy image than a blurry one.