Hi jin sun, the article you have copied is in slightly poor English so it makes it harder to understand, however, in this case" all fit" means that "everyone would have fitted into the boat" that is, there was space for everyone.
The article has other issues and gives us some clues, for instance, "twenty-five" tells me the author is American as in other countries it is spelt "twenty five". ponderosa pines should be spelt Ponderosa as it is a proper noun , while "Man, I had a blast" is colloquial.
"I wonder what it means here "all fit"." might be better phrased "I wonder what "all fit" means here?" The question mark is required to ensure we know it is a question.