Andaman Sea Blackwater

The extra depth of field is there to compensate for the camera not putting the focus at exactly the right point when the shot is taken. It's a buffer that accounts for the subject (and camera) movement.

Wrong. If you shoot in full focus, the extra depth of field provides a clear depth of field. Sharpness is lost as soon as the camera or fish is out of focus. the entire depth of field deviates from clarity when the focus changes. When using diopters like +5, +10, values like f25 are useful. Unless you're using a diopter, f25 just dims the entire image behind the fish. As long as you open the diaphragm, the whole image behind the fish begins to light up.

Let's see your blackwater shots taken at f/8. Mine are a blurry mess.

I am writing again, after f11, the light falling on all sensors is diffracted and the quality deteriorates. That's why none of the fish you put in is clear enough. You can't get quality photos at f25. Sharpness depends on shutter speed and the moment you focus. I'll find the f8s samples on my hard drive and put it here for you later.

No it won't, and believe me, I've tried. It kinda-sorta works in a limited range of apertures while shooting macro; all my attempts at using TTL with Retra strobes while shooting wide-angle ended up with extremely dark shots. Funnily enough, when I was using SeaFrogs ST-100 Pro strobes and triggering off the pop-up flash, TTL worked much better (but there was no manual option). With the Retras, I'm using manual mode on all dives except blackwater.

Then your TTL converter is not electrically connected. So you are triggering the slave with fiber optic cable. Flashes fire twice when TTL runs. It measures in the first combustion and gives the appropriate light in the second combustion. LED converter cannot do this. So your converter must be fully compatible with your flash. If it is not compatible, it means that TTL does not work, it just triggers. When TTL works correctly, if you shoot with f1.4 or f32, the light on the fish will not change, it will be the same. But when it works right. And while shooting the fish, you can change the aperture in seconds and shoot at any f value. All your photos come out perfect. Any overexposed underexposed does not occur. Between f15-f32, the back side of the fish starts to get dark. f25 shots in a very bright place are perceived as a night shot.

Are you speaking from personal experience or photo theory? If it's the former, please post some of your blackwater images taken with the settings that you recommend. If it's the latter, I suggest you take your camera on a blackwater dive and get some appreciation of the operational challenges involved. Your profile puts you in US; I don't know if there's much blackwater diving conducted in California, but Florida is one of the bigger blackwater hotspots.

What exactly do you mean by blackwater? Night diving, cave diving, hole diving, under the rock diving or diving in dark waters below 60 meters? I am a photogrammetry engineer and I have 25 years of underwater photography experience. All my equipment so far is professional grade. I generally used Sea&Sea systems. I had it done in special-custom products.