Here is what Sony says about A99 weather sealing

Operation doesn't always involve removal of the lens. The specs for "underwater operation", require not changing the lens under the surface. That is still operation. Besides you only use a UW when shooting underwater, so there is no need to change lenses. Changing lenses on a Nikonos is really more for use above the surface. People who shoot the Nikonos almost completely use the 15/2.8 if they can afford it. If not then the 20/2.8. The 28/3.5 is too narrow. All the other Nikonos lenses are for above surface photography. This is because of the close working distances required of the flash units underwater. Flash doesn't travel far underwater so you gotta be really close. If you want any FOV at all you need a UW while being that close.