I've been to the Hoover Dam twice and on the second time we took my father who is in a wheelchair. As far as accessibility goes there is different tours where you can get to see more of the facility but the regular one we decided to go to that was wheelchair accessible was good enough.
The parking facilities are also good (the structure on the Nevada side) where they also have accommodations for wheel chair spots as well as elevators, ramps and just about any other additional convenience to assist in touring their facility.
I have to say as far as accessibility goes the Hoover Dam was very thoughtful when putting together the accessibility and tour to accommodate families with members who are in wheel chairs.
The dam itself is quite a wonder and something to see. It's amazing to think how long ago this was built and all the perils involved in this work of ingenuity to finally become a reality.