@Barmaglot , you are correct - sorry, I meant to type 4" not 6" (went back and corrected it).
Well, there's at least part of your problem - the four inch dome is really too small to work with anything but a fisheye lens. While the 16-50mm will physically fit in it, even when you get it to focus, the image outside of the immediate center will be very soft. If you want to get good image quality out of the 16-50mm lens, you need to use the 6", or preferably 8" dome. I have done some pool testing with the various lenses and SeaFrogs ports that I own; you can see the results
here. The 16-50mm lens is absolutely capable of producing stunning images; these three were shot with the 8" dome, on the same dive, in the order of descending subject size:
Another point of note is that Meikon/SeaFrogs acrylic domes lack advanced optical coatings and thus are quite prone to flaring. Avoid bright lights, especially sunlight, hitting the dome edge-on - for best experience, keep the sun behind you.
Lastly, the images that you posted are suggestive of some sort of freshwater environment, perhaps a quarry? I have never dived in quarries, but with the water clarity that I can infer from your images, shooting wide-angle is a non-starter. I have successfully shot my A6300/SeaFrogs setup in water conditions where I had trouble seeing my own hands, but it was always on macro subjects. Here is a sample picture taken under those conditions, i.e. if you can't physically feel your buddy, you've lost them:
The little out of focus blobs are silt particles, and you can see sand grains liberally sprinkled over the sea moth itself. This was taken with a Sony 90mm f/2.8 G macro lens, at a distance of maybe 10-15cm from the port glass.