Given that a prayer can be used before ingesting water to remove its negativity, do we not really need to worry about the source of our drinking water if we use prayer consistently?

You can be reassured that this is the case. We are capable of protecting people from these sources of negativity, simply through prayer alone. The caveats are that no two people mounting the identical prayer will necessarily receive the same results. This is because of the usual requirements for having 100% belief in the divine, as well as belief in the person themselves to be worthy of divine support and assistance. Unfortunately, at the present time, many people fall short on both counts. So this is something needing reinforcement and enhancement. This can be built through prayer work as well. And this is why we advise a broad-based adoption of prayer in one’s life, because all prayer work is self-reinforcing with respect to raising expectations, raising belief in the divine through the doing itself, because this signals the subconscious that it is to be taken seriously and acted on with serious purpose and intention, all of which contributes to the empowerment of the prayer itself. So this will be a process, not an instant guarantee, if people are simply mouthing the words and expecting a 100% beneficial result. Like all aspects of interaction with the divine, there are contingent factors and obligations on the part of the human because every restriction comes from the human side. The divine has unlimited power. We can only use the power that humans allow and support, both.