I spent about 10 years living up around Tahoe. I do not have an underwater camera but managed to borrow a friends from time to time.
Shots 2,3 and 4 are the cast off boards that were not able to used as shingles. This spot near Sunnyside used have an old shingle mill way back in the day.
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The pic below I found when I was snorkeling one afternoon. So a few days later I went back with my gear and older film camera and water proof housing.
I took the pics over to Sierra boat company in Carnelian Bay and asked around. The on;y thing we guessed it could be is 1just and old work barge or 2 a target platform for gunnery practice for WW2 fighters back in the day.