It really depends on where you are going to be staying. Lake George is 35 miles long but two of the most visited areas are Lake George Village (South End) or Ticonderoga (North End).
If you are going to be staying near the Village then there are a few places that have some good diving.
First is Hearthstone State Camp Grounds. This is where most dive shops have their check out dives. This is a shore dive and depth is how ever deep you want to go. There are a few rock ledges and there is a car that is used to practice recovery techniques.
Next is Diamond Island. This Island is located straight across from Hearthstone. You need a permit and a boat to get to it, but it offers some nice diving. The West side of the island is a wall that drops from 20-30 to close to 70 feet. It offers some nice lake life and it one of my favorite sites. The East side is a very shallow dive around 20 feet and is where you could find some quartz crystals, which give the island its name. There is a sunken boat just south west of the island in about 30 feet.
To explain some of the wrecks that are located at the South End it is best to check out the follow link:
http://home.nycap.rr.com/sandon/lake.htm
If you are going to stay on the North End of the lake there is another site that is one of my favorites. It is called Rogers Rock or Rogers Slide. The slide is a cliff face that is several hundred feet high and drops into the water with a depth of around 80-90feet. It is a neat wall with many large boulders and ledges.
There are many other dive sites around the area, I suggest you visit our website and post this same question there. Our club president will be more then happy to give you some more information on diving Lake George.
Please get in touch with us as to when you will be in the area, I will try to get a few dives in with you.