One interesting thing about the "near vision" portion of my mask; Seavision grinds the "regular" vision on the inside of the mask so it really works just like a regular pair of glasses. For the "near" vision part of the bifocal, they grind it on the outside of the lens and they are designed to work underwater only.
This made me worry when I first got the mask because the regular vision worked great but the near vision at the bottom of the mask was blurry. A quick call to Seavision and they said don't worry, it will be fine underwater. They were totally right, as soon as you are underwater, the near vision part is very sharp and works perfectly.