The answer is no, Grandma Dowdel doesn't own a rowboat.
In A One-Woman Crime Wave, Grandma Dowdel takes Joey and Mary Alice to Salt Creek, where the Piatt County Rod and Gun Club has some property. At the edge of the creek, Grandma Dowdel uncovers a rowboat that's tied to a tree and hidden among some vines. The trio uses the rowboat to paddle out further onto the water, and eventually Grandma Dowdel stops the boat at a certain spot. She then plunges her rod into the creek and soon lifts up what appears to be a catfish trap. The crate is filled with "whipping tails and general writhing." In other words, it's full of catfish.
Although Grandma Dowdel admits that trapping fish is illegal in the state, she tells Joey that she's never yet gotten caught. Then, she admits that the boat she's using isn't hers at all. We later find out that the boat actually belongs to Sheriff O.B. Dickerson. When Grandma Dowdel rows past the stretch of the creek belonging to the Rod and Gun Club, the Sheriff spots the trio in his boat and yells out to them to "stop in the name of the law." Although the Sheriff desperately wants to write Grandma Dowdel up on charges, he knows that he can't.
The Sheriff realizes that he, his deputies, and the entire male business community would be greatly embarrassed if Grandma Dowdel ever reported what she saw when she rowed past the Rod and Gun Club: every one of the men had appeared to be stone drunk and many were in various states of dishevelment. Grandma Dowdel's testimony would have certainly upset some of the wives of these men. In fact, it was Earl T. Askew, president of the Chamber of Commerce, who warned the Sheriff about exposing them all:
"O.B., let's just let sleeping dogs lay. I got my hands full with Mrs. Askew as it is."
So, this is how Grandma Dowdel gets away with using a rowboat that doesn't belong to her.