The less affluent pay the same water bill. Yes, people are generally unresponsive to signs. Especially if they are facing the opposite direction as you say. And there are too many of them. Might be less expensive than dredging if your objective is to keep people from consuming contaminated fish.
People respond better on balance, to visual cues.
Could even have the message you want to convey on the bottomside. Should increase awareness. Responding the same way. Ray A RiskAnal colleague on smoking. Yes, Big Tobacco executives lie.