Whilst I fully agree that there are many good lenses out there by both manufactures of cameras and independents, thats not all there is to it. Often the mechanics of cheaper lenses are not as well built or robust as more expensive glass. But in a housing this may not be a real worry. I know an underwater photographer who has been using the same Nikon fit Sigma 50mm macro since the early 90s and he's still perfectly happy with it - great!
If you intend to use lenses heavily topside too then mechanics can be worth thinking about as can spares - a manufacturer who constantly updates and changes specs may not support lenses as long as one which builds the same lens for a long time. Also adjusted specs can be a real pain when it comes to setting up a lens with gears or ports.
There are other issues too, such as whether lenses, especially zooms, are supported by housing manufacturers - getting a zoom gear can sometimes be problematic for some lenses and whilst not insuperable (there are a fair few custom builders out there) this can take time and money.
Lastly, even camera manufacturers produce varying quality lenses these days. A cheap lens is usually cheap for a reason and an expensive one costs more because it is probably better designed and built, and digital cameras are, in my opinion, far less forgiving than film ones when it comes to showing up lens flaws.
