I was a long term volunteer there in 2002, and ran it repeatedly at these low levels. From personal experience, it was possible to take a 16' Avon Pro, a 16' NRS cat, and other such sized boats through the right side of Skull rock.
And you absolutely had to get your oars fully inboard of the tubes. The one time I wasn't quick enough nearly broke my arm, and broke the oar when it finally jammed between Skull rock and box on my boat.
The commercials were putting 18 buckets through there as well, but with a bit more difficulty. The picture below shows what would happen when one would get stuck in the slot and then fill up.