Water Heater electrical installation to breaker box
For a typical 240V tank style water heater you will need a 30A/240V circuit. No neutral is required. 

You CAN use the white wire as a hot going to the breaker for a straight 240V circuit such as this. You just need to mark it as a hot conductor with a permanent marker or such. You can use any typical hot color such as black, red or blue. You DO NOT need to use a red wire. 

Now....you ask "Can I install the red line to a second hot breaker". This implies there is a red wire in the cable. If this is the case then yes, you must use the red and you can just cap off the white in the water heater since it is not used.

You CANNOT simply use a "second hot breaker". You MUST use a two-pole breaker for a 240V circuit as previously mentioned.