What does Twain say is the one permanent ambition he and his boyhood friends shared?

This quote from Mark Twain’s memoir Life on the Mississippi comes from the beginning of chapter 4: The Boys’ Ambition.
Ambition is defined as a drive to achieve a higher goal for oneself. In Twain’s childhood, the ambition he shared among his friends was straightforward: “to be a steamboatman.”
Twain lists several other “transient” ambitions the boys had, including a desire to be circus clowns or pirates. However, the allure of the steamboat never waned among the boys. Twain states that this was due in part to the daily arrival of a steamboat coming up the river, which became the highlight of his small village’s day. Twain recalls everyone in town patiently waiting for the arrival of a steamboat, everyone clamoring in excitement once the sounds of an approaching vessel could be heard.
Therefore, Twain associated the steamboats with excitement from a young age. Steamboats were a gateway to adventure in the world beyond his town, an idea that was powerfully motivating to the young boys.
Besides this adventurous allure, the steamboats offered good wages for the various positions one could get aboard a boat. Twain remarks that two months' wages for a pilot “would pay a preacher’s salary for a year.” While pilot was the most coveted, lucrative, and rare of the positions, that fact likely appealed to a young Twain, because he couldn’t imagine what it would be like to make such a sum in a short amount of time.
Ultimately, Twain—and his friends—regarded the steamboat as an escape from the humdrum, provincial life in which they were raised. This created a lasting appeal in their impressionable young minds.