Your main issue will be the water temperature. It's been mentioned the water there is 42 degrees. That's cold! I was in 45 degree water...once...and nearly froze. And I was in a dry suit. Anything under 68 degrees is cold to me, though. 
I believe Castle Lake is a fairly shallow lake. At 2500' the effects of the altitude are minimal in shallow depths. As long as you remain conservative, there shouldn't be much of an issue. However, if the lake is at 7000' as has been stated, then it becomes more problematic.
I suggest you google altitude diving. There are a few articles out there that go into some detail on it. You should also be able to find a conversion table online for depths at altitude. More than half of my diving before I moved to Florida was altitude diving. There really isn't much to it.

I believe Castle Lake is a fairly shallow lake. At 2500' the effects of the altitude are minimal in shallow depths. As long as you remain conservative, there shouldn't be much of an issue. However, if the lake is at 7000' as has been stated, then it becomes more problematic.
I suggest you google altitude diving. There are a few articles out there that go into some detail on it. You should also be able to find a conversion table online for depths at altitude. More than half of my diving before I moved to Florida was altitude diving. There really isn't much to it.