That's an idea - a barrel full of fish, but not full of water. Though I can't see people saying "fish in a barrel", not in English - they'd be likely to say something like "a barrelfull of fish", the same way they say " a barrelfull of monkeys".

Unless the original phrase might not have been English at all, but imported along with the fish from some country where that way of putting it might be more natural. Norway? The Netherlands? Germnay? Or some community in the USA where that speech pattern might have been carried over at he start of the last century.