Steamboats:

The young Sam Clemens learned all about piloting a steamboat up and down the Mississippi River and would have stayed on with that profession had not the Civil War made his career choice a bit dangerous for his taste.

Clemens' pen name Mark Twain is steamboat lingo for two fathoms(12 feet) of water, which means the pilot can journey safely on the river without running the boat over shallow water or sudden land croppings.