What is the purpose of the

I am considering purchasing a wide angle lense for my setup - but notice that alot of the profesional shooters utilize a dome port (such as the fisheye). Any insight on what the difference between the dome port vs the flat port?

I'll take a shot at this one.

Fisheyes often work well underwater because you dont usually have the hard vertical lines that you find on land, hence you do not get the "fisheye" distortion as much underwater.

That said, a non fisheye lens such as a 15mm Nikonos or whatever will still give you the most pleasing images for your set up. What you will find out is the wider you get the more you will get barrel distortion, or curvature around the edge of the image.

As far as ports, A dome port is usually required for a wide angle lens due to the angle of the image. A 15mm wide angle lens on a SLR will have a field of view of appx 100 degrees. A flat port will usually stick out far enough from the housing that the image will be blocked. The dome port creates an "apparent image" which allows the full frame to be captured. Basically it creates an apparent image about 2x (i think) the radius of the dome port in front of the port.

I believe their are some high end housings that will make a flat port for ultrawide (weitwinkel) lenses, but I am not positive about that. I always used a dome with and ikelite housing and added a close up diopter to correct for the apparant image. Worked like a charm!