Where do fish keep their money?

“Ocean”

While fish live in the ocean, they do not have a concept of money or a place to keep it there. The riddle uses 'riverbank' as a play on words.

“Underwater cave”

An underwater cave is a plausible location for fish but does not relate to the concept of a bank or storing money, which is the play on words in the riddle.

“Coral reef”

A coral reef is a habitat for fish, but it does not have the association with banking or money storage, which is central to the riddle's humor.