Responding to the other thread about settings/lack of and so on i might as well post when i do on a typical photo dive. Note the things i mention are an average - nothing is constant. Starting points only!
I'll base it on a house reef dive abroad settings wise but it doesn't really matter. This is for wide angle and will certainly not work for macro!
Firstly, before entering the water ive got ISO200, 1/160th shutter, f/8, strobes with diffusers out to the sides, diffusers on set to full power. Lens is zoomed in fully (so 22mm on my setup). Camera LCD brightness is low (more later), set for centre point "one shot" focus.
The reason for that is if something big swims close(ish) and quickly before im ready ive got more than a fighting chance of getting something out of it that i can rescue in post processing. To date though nothing ever has so i can't really back up this information!
Then as i descend im taking photos of nothing in particular. Buddy's fins, blue water, a rope etc. These shots are allowing me to keep the background colour i want as i descend and light levels change. Again this means if i DO need to take a snapshot the settings are kind of right(ish). I do this a LOT throughout a dive, just snapping pictures in the direction and depth im swimming in between photo stops to keep a decent ambient exposure dialled in. All the time im swimming along im looking where the sun is (assuming its visible) as its important.
When i get to an actual photo subject (assuming it doesnt move much) i usually just take a shot and see. Background should be roughly OK from above, unless its something fancy like CFWA or directly into the sun f/8 should be OK. After that photo im looking (i) was there scatter (ii) are the strobes properly lighting the subject coverage wise? (ii) is the strobe power correct? then i adjust and go again. Sometimes this takes 1-2 shots, sometimes 10+. To me its all trial and error.
*ROUGHLY* on my setup on that reef at the middle of the day at that depth i know that usually f/8 (UWA lenses have a big depth of field) and strobes with diffusers on about -1.5 stops will result in a near-enough foreground exposure so i try that first (unless its white or black etc).
As i move around my target im aware of the sun position as this means im going to need to change my shutter speed depending on where it is. If its extreme and a low sun angle i might need to adjust aperture and iso as well (and therefore strobe) but i try to avoid this - its more work!
I'll admit on a wide angle dive i very rarely if ever adjust the ISO (200) or the aperture (f/8). Yes if im doing CFWA or its a dim afternoon yes but for a lot of the dive im playing only with shutter speed and strobe power. Even strobe power doesnt change THAT much as long as its an average subject and the lens stays on the same focal length so in reality shutter speed gets changed a LOT, strobe power a bit less, aperture and ISO much rarer.
Looking through my house reef dives and my shutter speeds on average range between 1/100th and 1/200th (max sync speed). apertures are going from f/7.2 to f/13 and ISO 200 or 400. The majority are f/8 and 1/160th. But thats because its a shallow sandy reef in the red sea 20m off shore and usually 8m deep! It'll be meaningless for elsewhere - every site has different characteristics.
I think it's easier to limit the amount of adjustments you make at any one time to limit work load and the chances of error so i work on the basis of keeping as much the same as possible and changing only one thing (although clearly there are situations where this isnt possible. Many situations).
So while im swimming its keep taking shots of nothing to get a background. Do this when you change depth and sun angle so its always kind of right. Be aware of where the sun is (this is actually easier on overcast days as the sun angle isnt an issue). With a strobe shutter speed should ONLY affect your background whereas ISO and aperture will affect both so try if possibly to play with shutter adjustments first and only delve into the others if needed (as it means you'll need to alter strobe as well).
Why do those settings work for me? Well they're found by trial and error because its a reef ive dived literally several hundred times so its just based on that. Middle of the day vs late afternoon (i dont "do" mornings...) can yield up to 2 stops difference in light level. Into and away from a low sun can be even more and so on. Just rules of thumb based on nothing more than repetition.
Ive not done THAT much UK wide angle but i usually aim to start around f/5.6 and 1/40th ISO 200 and work from there. I've found UK light levels vary far more than abroad regarding depth and so on so theres a lot more fiddling to get something right - my settings are still based on the result of test shots on descent and during the swim. If anyone wants it - ive got a really good collection of badly exposed sets of fins (various brands), backsides or rocks !
I'll admit to not knowing much about this at all - even less than wide angle.
Generally the principle of trying to limit what you change comes into play again. If you dont care about blue backgrounds or background color too much you can simplify things here by just setting the shutter speed to 1/200th or whatever your sync speed is (within reason on compacts) and it'll be black. Then its just aperture to adjust next. This is depth of field so pick that, set it and (generally) leave it. I start at f/11 but have used anything from f/2.8 to f/22 depending on the effect i want.
Once you've done that put your strobe(s) where you want the light angle to come from (think of it as an artificial sun) and take a test shot, look at it, look at the histogram, adjust the strobe and repeat until you get the shot. Obviously it helps if the subject doesn't move. Pointless squishy lumps of toothpaste like nudibranchs are easy for this (fwiw i class anything finger size, squishy, uninteresting and slow moving as a nudibranch regardless of gills).
If you're afraid the subject WILL move do the above on a lump of rock a similar shade to the subject a meter or two away. This tip works for wide angle too.
OK so thats a lot of waffle. As i said all the stuff there is general in that its my starting point but probably a very small percentage if any of actual keeper photos from those dives will be with those exact settings - they're just rough starting figures to shoot and adjust FROM.
Oh, why is my camera LCD set to dim? Because in low light underwater it appears really bright making under exposed photos look fine. Yes you should use the histogram as well but its a rough indicator and less confusing if its not totally contradicting what the histogram says.
All the above is based on a crop sensor DSLR camera with DS160 strobes so wont be directly applicable to settings on compacts. The method should be though. And yes ive missed lots out but its designed to answer a question elsewhere about general guides for a typical dive.
Feel free to shoot holes in it.