Why would the fishermen not be the optimal finding source? Basically the premise here is that their use of plastic nets is already externalizing a cost onto society. Forcing them to pay for nets that cause less externalities just means we're moving the cost away from all of society and back to those who are benefitting from it.

You're assuming that they're the ones benefiting from it.

Suppose they use plastic nets and that makes fishing less expensive, but they all do that and it's a commodity market so they don't get to keep any of that money, it just means that it reduces the price of fish.

Now we need them to use more expensive nets. If we put the cost of them on the fisherman, it raises the price of fish, a healthier source of protein than many of the alternatives (e.g. red meat), and raises the price of food in general. Not super great as an outcome.

If we put the cost of them on coal companies, it raises the price of coal. Coal has nothing to do with fishing net, but we're actually trying to discourage coal, so since taxing coal burning is better than taxing fish eating, that may be a better choice.