Castle Lake, up by Mt Shasta in northern CA.
I planned to dive this lake right as the ice layer was partially melted at the deeper end. Unfortunately, overtime at work, delays in getting gear serviced, and a 90F heat wave through parts of the area meant I was about 2 weeks late for that. The water was still cold (dropped into the 40s past 20ft), but now also rather blurry with poor vis. A few other minor issues compounded into a dive that just wasn't as enjoyable as I was hoping and I had to cut it short at around 30mins. I might try again some other time.