Little Tim wants to be a sailor, but his parents tell him he's too young, even though his retired boatman friend has taught him all about ships. He decides to convince them by stowing away on a steamer. When he is discovered, he is put to work scrubbing the decks, which is a lot harder than he expected.
Here's where it is important to realize that this book is actually from the 1930s, not the 1980s as most of the entries indicated (I added this is a new edition, hopefully the correct date will now show up). If this were a modern publication I'm sure the point would be something about Tim missing his parents and realizing running away was a bad idea; he would soon be taken home and reunited with his parents, and gladly return to school.
But this book comes from an era when many children left school for work not much older than Tim (he looks about 8). After the initial few hours of labor, Tim adjusts to life at sea. The sailors are pleased that he works hard and doesn't whine, and teach him lots of useful things. He returns home only because the ship sinks in a storm. When the captain tell his parents how hard working and brave Tim was, the agree that he can become a sailor after all.
My copy of this is from 1955, and describes itself as "completely redrawn with additional text."