Welcome. My name is ***** ***** would be glad to assist.
If the feed now is 240 volts for the electric water heater, it only has one extra wire, which would be the ground.
In order to supply 120 volts to operate a light, you need a neutral wire, which is basically the common side for the 120 volts.
One hot phase and a neutral will produce 120 volts.
The option you would have at this point, is to either run a new circuit for the light only or you could add a small transformer, which would be connected to the 240 volts on the input side and it would produce 120 volts on the output side.
Unfortunately, there are no other options, since you do not have the extra needed wire form the panel.
Let me know which direction you would prefer to take and I can assist on the addition with the details of the wiring etc.