You must re-mark the white wire
You need both hots and ground, but you don't need a neutral. So what you need for that job is red-black-bare cable.
Unfortunately, that cable is not made.
So what you actually need is red-white-bare cable, and some red tape. You will re-mark the white wire red, using red electrical tape.
You must re-mark the white, but to tell you the truth, it doesn't need to be red. It can be any legal color for a hot wire, which is any color except white, gray or green. Black and black is perfectly fine. Feel free to use black tape instead.
Now, on a 2-pole breaker, you have 2 terminals for hot wires. And by golly, you have 2 wires that are hot colors. You can figure it out :)
We should see other metals
One more thing. If you are about to spend hundreds of dollars on long runs of wire, stop. You should price an alternative: heavy aluminum feeder wire at a tiny fraction of the price.
First, you can definitely run large aluminum (1 AWG or larger, 100A) to a subpanel near the water heater. In the subpanel, place your two 50A breakers and run #6 copper the short distance from the subpanel to the heater. You could also upsize the aluminum wire, to bring more amps to the subpanel - either for shorter wire runs for additional circuits, or for a larger water heater.
Or second, if the water heater's terminals are rated for 75C thermal, and they are also rated for aluminum wire, then you can simply replace the 6/2 copper with 6/2 aluminum wire, which is good for 50A.
That seems illogical, how can the same size aluminum wire do the same ampacity? It's a fluke. Because of the NM or UF copper wire is an inferior insulation type limited to 60C thermal, so 55A. Aluminum wires in the US are made with better insulation good to 75C thermal, so 50A.