The phrase from а book.
" ..it would be better for him to stop in the boat."
What happens if I replace "to stop" with "to stay"?

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@dmitrypo1977 Fascinating. If I understand correctly, it is meaning that it is better if he stops what he was doing and goes into the boat to make tea (briefly, before returning to his previous activity afterwards).

It's perfectly valid, but it reads slightly awkwardly to me. I might have put in another preposition after the 'in' to make it less ambiguous, such as 'to'.

Specifically, to stop in somewhere, meaning to briefly stop what you have been doing, in order to visit somewhere. I thought the 'in' (after 'stop') was simply a preposition, but it is actually part of the verb "to stop in" (here's a definition https://idioms.thefreedictionary.com/stop+in+so... FWIW).

The fact that you can swap 'stop' with 'stay' and still get a grammatical (and sensible) sentence, is a bit of a fluck (coincidence) to be honest! The intricacies of the English language I guess.