If you were to take a piece of mirror that is flat and look at it, you see a really limited range within the borders of that flat panel. But, if you were to take that panel and wrap it a bit you get a much wider range within the same borders (though reflected in the mirror). The same concept is present in the Hydroptix mask.
Think about the magnification concept of a mask. Objects appear closer and larger than they actually are with a standard mask. This is based on the characteristic differences of that image being transferred through water, a lens, then air to get to your eye; three different mediums which have different visual transmission characteristics. When you dome (convex) a lens, you increase the air mass capability within the mask as well as the magnification characteristic of the lens. Thus light bends at a different angle than with a flat lens causing a better field of view. Does that help clear things up a bit more?