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I'm thinking about a somewhat shallow dive as well.
Crater Lake at Crater Lake National Park in Oregon is a beautiful high altitude dive. I've hiked in and out of it a number of times and have decided not to dive it myself, but I know those that have.
Clear Lake, outside of Sisters, Oregon is another high altitude dive in the Oregon Cascade Mountains. One does need to be careful about setting your computer (or high altitude calculations on your tables). I do this dive every year in a 7 mm wetsuit, but almost everyone else is diving in their drysuit.