Rev. Blade:http://www.hammerheadcove.com/ is supposed to be in Shasta Lake City .
I suppose if I was a bit more on the ball, I'd reach out to the ski club that mentioned them as the shop they use when they dive (in the off season) http://www.weski2.org/
Hammerhead Cove is a new shop in the area. I wasn't familiar with them. A benefit that they are filling nitrox. As Peter C. mentioned, Whiskeytown is the lake that is more often used for diving in the area. I have done quite a few Whiskeytown dives and it is a nice place to dive.
You will probably want to stake out a good place on Lake Shasta to park the houseboat. Cruising around too much will eat you alive in gas $$ and you risk losing your nice beaching spot. Hopefully, you will have a runabout of some sort to get you about the lake and to go pick up supplies if you need them. Have fun!