Beware of using water analogies to try to understand electricity, the equivalence is limited and in many cases just plain wrong.
In your situation there is one source of EMF in the circuit, the 1 volt battery.
So there can never be more than 1 volt in that circuit.
You have wires with varying resistance, in the circuit as drawn this does not matter, there is no load and hence no current flowing, so there can be no volt drop in the wires.
The potential difference a points A and B is exactly the same as it is at the battery terminals ie 1 volt.
However if there were to be a load at points A and B such that a current flowed between them, then the potential difference between A and B would be less than that at the battery terminals.
What that potential difference would be we cannot calculate because we do not have enough information. We would need to know the resistance of the wires and the resistance of the load.
JimB