[KenByrne]

It's difficult to suggest a single lens for wreck photography. The first decision is whether to go with a rectilinear lens that will give a fairly undistorted view up to about 100 degrees or a fisheye which gives a view of up to 180 degrees. I use a Nikon 12-24 at the moment but find it's a little soft at the corners behind the Ikelite Dome port. There are alternatives to this from Tokina and Sigma but watch the size they can be fairly big and can't fit in all housings. For fisheye the Nikon 10.5mm is a great lens and is fast at F2.8. I'm waiting for delivery of the Tokina 10-17 fisheye zoom which is cheaper and more flexible but slower at F3.5 maximum aperture, another disadvantage is that I understand it doesn't take filters.

When it comes to ports generally speaking macro lenses, which you really need for fish pics, are best used behind flat ports and wide angle lenses need dome ports. If it fits you can use a macro lens behind a dome port but you lose some magnification. Unfortunately you alos need a port that is the right size for your lens. This is acheived either by buying multiple ports, which tends to be the Ikelite way, or one port with extension rings. I use an Ikelite housing with multiple ports but these tend to be cheaper than the extension rings of some manufacturers.

As I've already said I use Ikelite. I'm happy with the functionality and particulary the price of these housings. A bonus is that if you buy Ikelite strobes as well you get ttl flash support. Some other housings are smaller and lighter, not always both. If you decide on a specific lens make sure the housing you select accepts that lens. You also need to make sure that the housing you select allows you access to all the controls you are going to need underwater, some provide less controls than others. For example a quote from the specs for the Sea & Sea D80 housing specs is 'Controls almost all of the Nikon D80’s essential functions underwater.' before I bought one I'd like to know what I can't use underwater.

Sorry this is a bit rambling but there's a lot to cover in your question.