Dave,
In general the "best" Lake Tahoe dive spots are accessable by boat.
The most common / popular place to shore dive at Lake Tahoe is at Sand Harbor on the Nevada side. It usually has a number of divers, including classes. However it is not very deep. It was about 44 feet within the main area. Some small fish, and lots of crawdads. The occasionally trout is seen, but not yet by me. Lots of sand and big boulders.
If you continue out a bit further, it does start to drop of pretty dramatically, as does the temperature. Since Sand Harbor is shallow, it tends to be shall we say, not as cold as other areas.
Note that Nevada law requires divers to use a dive flag, so if you swim out past the roped off area, you are supposed to have a dive flag.
My buddy got winded walking fully geared up from the parking lot to the water. After the second dive he breathed some of his remaining Nitrox on the walk back to the car and he was much better.
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Rubicon Wall is on the California side, so a dive flag is not required. James (fdog) described the details, here is the overview. The way I hear it, there are two ways to get to the wall dive site 1) Park in the lower lot and you have a loooong swim, 2) park in the upper lot and you have a long steep hike but a shorter swim. It depends upon which you dislike less. (again, leftover Nitrox can ease the hike / walk back to the car!)
I hear that the wall is cool, with some good sized fish, and LOTs of crawdads. Not much else though.
BTW, when we took a class at Lake Tahoe (at Sand Harbor) the Instructor showed us something interesting. He took us to a granite rock the size of a small car. The rock had a sharp pointed ridge. Nearby was a large plank (10 - 12 feet long, 2 x 12 inches). He set the plank across the rock ridge, and put a diver on each end. It made for an interesting underwater teeter-totter. It took a bit to get it working right, but it was good practice trying to change your bouyancy as needed just through breath control. The tricky part is timing with your buddy. As soon as the teeter starts to totter, you each need to inhale / exhale to slow it then reverse it. Once you get it going it is funny to watch. I just wish that I had a video of it.
Also, when he picked up the plank, there were at least a dozen crawdads suddenly exposed, as they tend to congregate under the plank.
Just some thoughts at random,
Wristshot