Dome vs. flat port for Nikon 8400?

I just got the ikelite housing for the Nikon 8400. So far I'm really impressed.

There is an optional dome port for this housing to use with the WC-E75 wide angle lens. Am I correct in my understanding that with the flat port, the 24mm FOV of the camera is lost due to diffraction? If I get the dome port, will that restore the lens to the 24mm FOV I see on land?

For wide angle definitely get the wide angel lens and dome. This combo limits your macro opportunities so use the flat port if you are at a site that is all close up or macro opportunities.