Final answer:
Mark Twain dreamed of becoming a steamboat pilot, an ambition detailed in his memoir 'Life on the Mississippi'.
Step-by-step explanation:
According to the readings, Mark Twain, or Samuel Langhorne Clemens, dreamed of becoming a steamboat pilot when he was a boy. This aspiration is vividly depicted in his memoir Life on the Mississippi, where Twain shares his experiences training to pilot a steamboat on the Mississippi River. His pen name, Mark Twain, itself is a nautical term signifying a water depth of twelve feet, considered a safe depth for steamboats.