“never worked a day in his life”
Read “Life on the Mississippi. He didn’t just work on boats, he was a riverboat pilot (hence the pen name Mark Twain). “Life...” tells in detail how you become a riverboat pilot, which takes years of practice to get good at. And requires brains and an incredible memory. And it’s really hard — four hours on, four hours off.
Twain could be unpleasant, especially after the deaths of two of his daughters, but he was no slacker.
One had to remember where all of those sunken (and deadly) snags were, as well as sandbars, shoals, the constantly-changing riverbank itself (remade with every flood and storm), etc.
... for where all the "snags" and shoals and treacherous hidden currents lie.
And, THAT could change, day to day!