I would guess that Mark Twain saw the passage from the Overland Monthly and 
hung on to the idea, maybe subconsciously, until he was working on Roughing 
It. But aside from the idea of floating above the water on a balloon and the 
clearness of the water, I don't see any resemblance between the two 
passages. To put it in "Twain-ish" terms, the first is like a lightning bug, 
the second is like lightning.