The first idea the fishermen had was probably the same one that anyone would have had in this scenario - use a fishing net to pluck the cubs out of the water.
It's a nice idea in theory, but that's not what fishing nets are for. Nobody ever designed a fishing net to cope with the weight of a bear, and nor do bears understand what they do. The bear didn't seem to grasp the idea of grabbing onto the net, and the net wasn't able to take the weight anyway.