I have been diving there about a dozen times, but not this past year. The quarry is on two sides of the road--the "swimming pool" side, and the fishing side.
Over the last few years, rules about diving there have changed from time to time--years ago you had to sign up downtown, then at one point you could pay and sign in on the "swimming" side, then the "fishing side" was posted as "no diving" and locked, and then there were no signs or locks. I don't know what the current rules are about diving there.
The "swimming pool" side is not usually open for diving, but I was on a club dive there a few years ago. There is some junk on the bottom (e.g. a bike), lots of pumpkinseed, and some carp. It is up to about 25 feet deep, and viz is up to about 10-15 feet, depending on the season and weather.
The "fishing" side of the quarry is similar depth. There is a short length of rail on the bottom. Lots of fallen trees that are an entanglement hazard. And some fishing line and fish hooks. The end farther from the entrance is shallow and weedy. In late summer, there can be a lot of algae.
There is a tunnel that runs under the road, connecting the two sides of the quarry. It is partly collapsed, dark, and full of rocks and rubble, so it is dangerous.
Overall, better bets for local diving are Sarnia (if you have experience diving in fast current and someone local to show you around), or Trout Lake at Innerkip (if it is early summer and the weather has been good), or the scuba park in Welland, or Sherkston Quarry.