Dome Ports - Why use them?
Most underwater cameras are designed to be used fully underwater. For GoPro HERO cameras, the flat lens cover on the front allows for some fairly sharp underwater video and photos.
However, if you hold the camera half in and half out of the water, you get a differential between the underwater part and the above water part. The underwater section usually appears out of focus.
Using a dome port corrects this and allows you to achieve some pretty cool effects, with both sections of the image in focus.
It takes some getting used to and a fair bit of trial and error but the results can definitely be worth the effort.