There is a layer of water between the port and the wide angle lens.
The lens remains a wet lens, you can reduce that layer but still there is water in between so you can't consider the lens as if it was enclosed with the camera in a dome.
So you can't extrapolate field of view as you did as that will not be correct
The water layer between the rear element of the wet lens and flat port is constant with or without the dome port, their distance remains the same, so the optical effect caused by the water will be constant.
Since these are afocal lens, it will present a smaller image to the flat port/camera's lens. If your camera has a zoom lens with its shortest setting at 50mm (35mm equiv), it will not be able to capture the side of the presented image, thus you will have a narrower field of view of maybe 130 deg above water. I have the UWL-100, which is described as having a 158 deg field of view above water, but looking through it, it is obvious that I can see objects directly to the side of the lens, but the combination of its exit pupil, the camera's lens distance to the rear element and my camera's widest zoom setting, the specified field of view would be only 158 deg.
Lenses like the UWL-100 28M76 has a larger rear element, thus a wider exit pupil, which would be suitable to cameras lenses with a wider field of view such as a 28mm lens, or ones that does not use internal zooming, thus causing the front element of the lens to be farther away from the wet lens when zoomed wide.
Well, Inon just release another wide angle lens, the UWL-S100. This is design for zoom camera that's wider than 28mm. The description is not very clear as I believe it's translate from Japanese. The FOV is also 100, but with a dome, its FOV will be 150 underwater. So this will be similar with the UWL-H100, however, I'm wondering what's the weight of the UWL-S100 + dome. So would the UWL-S100 solve all of you guys problem?
The UWL-S100 is made for superzoom cameras that does not use internal zooming, So when your camera is zoomed to full wide, you will get circular fisheye effect given the UWL-S100's small pupil exit diameter and the far distance of the front element of the zoom lens and the wet lens. In order to eliminate that effect, one has to zoom the lens to 80mm (35mm equiv), so that it can capture the full image presented by the wet lens.