9 hours ago, tailwind_marseille said:

Regarding what to shoot, I want it all ! :) Really, as a newbie it's hard to say. It's early to want to specialise in anything. The options we have here in Marseille are enormous, there's big fish (merou, barracuda, tuna, etc), large schools of many species, nudibranch everywhere, corals, and many wrecks. I've done night dives in wrecks that I wish I could repeat the conditions with a camera in hand.

I do seem to be attracted to wide angle and large apertures in my topside photos. I'm thinking it will be the same underwater.

What focal distance would you be thinking about when wanting to shot mid sized fish ?

Well, I would suggest that you base your selection on the availability of lenses and ability to easily add more lenses as time goes by without requiring a camera system change and the consequent new housing.

UW is quite different to land based photography. using wide angle lenses up close to reduce water between you and the subject. I tried for the first year or two to apply some land based techniques and found they didn't work so well. Placing a lens in a dome port doesn't make it just like on land , it's necessary to stop down to deal with the aberrations caused by dome port optics, most people shoot around f11-13 for wide angle behind a dome port, even fisheyes are generally stopped down. Some of the wetoptics can be opened up a bit more.

Typically a flexible solution involves a zoom lens, you can swim right up to a coral reef, but if you try doing the same thing to a shark or a tuna it will swim away. SO being able to zoom from a static subject to something with some more reach is very handy. By more reach it's something in the range of a 30mm equivalent lens on full frame. It's still significantly more reach than a fisheye.

Wide apertures are used in macro work, where you can actually create some good bokeh, in wide angle work it can create some rather ugly out of focus things in the corners. Here is an example of photo taken 17mm and f8 in a small dome, note the lower left corner:

https://uwaterphoto.com/?p=839

For fish portraits a longer zoom or a short macro lens tend to be good options.

Again I'd suggest rather than gravitating to a camera body go ahead and see what you would need for macro, mid range and wide in a couple of different systems, then see how much it will cost you and the weight and size. I don't know if you plan to travel for diving but a compact system makes traveling a lot easier.

You may not like the fisheye perspective for wrecks, that is down to taste.

A factor to consider is that behind a dome port you'll have to stop down to get decent sharpness in the corners. There is plenty written by far more intelligent people than I am about why.