A 220V what, cable or heater? I assume you meant cable.
Doesn't really matter, you are just a little confused. An electric cable is rated for a certain amperage and certain voltage. The size of the wire determines the amperage it can safely carry. The insulation determines the voltage. The way the wires are connect in the breaker panel determines the voltage to the appliance.
The service to the house by the power company determines what voltage you have. At one time, a long time ago, service in the US was 110 and 220 volts. That was a long time ago. Service was later increased to 115 and 230 volts, then to 120 and 240V. So you have 240V service. People just can't seem to drop the "220" description.
All residential cable is rated at 600 volts. The size of the cable would be for 30 amps unless the water heater is at an unusually great distance from the breaker panel.
Most residential water heaters require 30 amps at 240 volts. You have 240 volt service. The cable to the heater should 10/2 with ground. The 10 is the size of the wire in the cable and the 2 is for 2 wires. "With ground" means it has an uninsulated ground wire for equipment grounding purposes. You only need two hot wires for 240V to an appliance because a neutral is not required. The connection at the heater is from hot to hot.
Look at the cable. Assuming Romex cable it should be marked "10/2 with ground 600V"
120V means 120 volts to ground or neutral, 240V means 240 volts between two hots which are in opposite halves of the cycle.