one other note: "going wherever the river flowed" is kinda a saying, in english, it would be "going wherever the wind blows" implying that he just drifted around, not that he's actually taking the river.
but the marisa is stupid and takes it literally, so I can't reword it.
Humm, maybe "go with the flow"? Something about "ebb and flow of history"? I'm probably missing a better one, somehow "current" is ringing around my head, but I can't quite put my finger on it.
Maybe: "One-inch exorcist always let the current carry him." "Marisa will let mr. river current carry her!"
It's not as popular a saying in the West, but "going where the river flowed" does not mean the same as "going where the wind blows".
The former means that the person (in this case, the one-inch exorcist) lived a righteous life as a member of a community (the flow of the river referring to the state of society). The later means that the person is living a directionless life, going where the whim takes them.
As the one-inch exorcist is a direct reference to the actual Japanese myth of the one-inch samurai, the former would be more applicable than the latter. The original fable itself is full of symbolism and lessons to impart to the human children who were the target audience.
Of course, in the context of this story, Marisa wouldn't understand the concept of figurative imagery, let alone symbolism, and so took the story literally.