50hz water heater used in 120/208v 60hz system

Whats the situation?
Someone did a repair with that issue?
Someone sold you a water heater with that issue?

50hz is usually designed for 220v European electricity supply, so in that case it will be getting less power than what it was designed for - which will likely make it less efficient..... (since you can assume the design engineers make it most efficient for the voltage it is rated for)

50 or 60 Hz has nothing to do with a water heater. I would be very surprised if your heater actually had such a rating. I am a little surprised that any such question comes up at all because it is very rare to have both 50 and 60 Hz power in an area small enough that you would bother to carry a water heater from one place to another.

In any case, a water heater usually costs less than its installation, so don't hesitate to buy a new one if you are uncertain about anything. The tank contains an "anode rod" which protects the tank by being consumed, and it usually lasts as long as the guarantee. You can have the rod replaced at the end of the guarantee period and that makes the tank last that much longer. But since you are installing a heater, there is no point in doing that. Just buy a new one.

They would not have 50hz and 60hz power supplies in a small area.

50hz is common in Europe and 60hz is common in America.

Its very easy to make the mistake to buy things on Ebay etc with the wrong power requirement if you did not check very carefully!