Hi Stefan
It really depends on your lense, port, diving conditions.....
I have the RDX600D housing with standard port and recently bought the compact dome port too. I have the standard lense that comes with the camera as well as a 60mm macro and Tokina 10 - 17 fisheye. I have a Inon z240 strobe and a tiny (old) YS17 (LOL). Also a focus light which is really important if you want your macro lense to focus!
As far as settings goes, it depends on the water conditions, what you are shooting and what equipment you are using (lense type, strobes etc). If you can do yourself just one favour, purchase Martin Edge's The Underwater Photographer book (4th Edition) - it is the best thing you can do to learn how to take photos underwater.
Just back from a dive trip to Mozambique, I found I mostly used the standard port with the standard lense due to its versatility (you can get close to small objects and fit bigger objects in too). My settings (with strobe) were ISO 100, Manual, Auto white balance, F14 - F18 depending on what I was shooting with strobe on TTL. Camera speed depended on the background I wanted - 1/150 for a darker background and 1/60 for a lighter background. I used a lot of settings and it is best to experiment as much as possible.
You can check out some of my photos in my 2013 Mozambique Trip album on my facebook page: Indigo.ScubaSA
The best way to do all of this is get the book, understand ISOs, shutter speed and aperture and how they all work together - that is the biggest part of the battle won. And enjoy it, don't stress
