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Depending on the type of wire, you are marginal in your wire size. A water heater is considered a "continuous load" and needs conductors sized to 125 percent of the nameplate rating. At 9Kw, this comes out to 47 amps. 8AWG is OK only for 75 degree (C) rated wire and 75 degree rated terminations (the screws on the water heater and the circuit breaker). UF wire is NOT usually 75 degree rated. You need two "hot" wires and one equipment grounding conductor. The hot wires can be any color except bare, green, or white. These will attach to the power connections on the water heater and (one each) to the two screws on your two-pole circuit breaker, through which they will, in turn, be connected to L1 and L2 from the power company, thereby delivering 240 volts to your water heater. The equipment grounding conductor will connect to the grounding terminal on the water heater and the grounding bar within the service entrance panel or subpanel. The equipment grounding conductor must be green or bare. It is a code violation to use the white wire as an equipment grounding conductor. At 8AWG, you do not have the option of coloring the white wire green or stripping the insulation off to "turn it in" to a ground wire. If you are connecting to the subpanel, you need to understand the difference between ground and neutral. They are NOT the same. On the other hand, service entrance panels normally have the ground, neutral, and grounding electrodes conductors all bonded together (this is the one place they are allowed or required to be tied togther). Your completed water heater circuit SHOULD NOT have, or be connected to, a neutral wire. The white wire within your cable should not be used.
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