It's not necessarily redundant, just clarifying direction, based on an alternative definition of the word which emphasizes the movement of the subject without necessarily implying that the object is water by the verb itself.
According to Oxford dictionaries, an alternative definition of "dive" is:
move quickly or suddenly in a specified direction:
Example: "a bullet passed close to his head, and he dived for cover"
Synonyms: leap, jump, lunge, launch oneself.
It's likely that Boas is merely using dive in this sense, and thus manually clarifying the direction ("down") and destination ("the river") of the action taken by the subject. The subject could, by this definition, have dived sideways, or even upward, and still come to rest in the river.