Does a 240v water heater circuit require a neutral?
Confusion from failure to use marking tape
Yeah, I know.  Marking wires with electrical tape to indicate function seems like such a needless chore.  &quot;I only need to understand it right now.  I will never be in here later troubleshooting!&quot;  This of course ends up not working 1/4 as well as expected.   Some of the people who are marking-hostile write installation instructions.
Since UL approves installation instructions, I'm surprised they don't make the instructions follow Code.  Because in this case, Code sides with me.
It is illegal to use white as a hot.
However, if you are buying pre-manufactured cable, what you are allowed to do is use paint, tape, shrink tubing, etc. to re-color a white wire to be an appropriate hot color.
But it's mandatory.
So the most straightforward answer is to take your #8 supply cables, and after you're done stripping off the cable sheath to fit, go ahead and wrap the white wire with red electrical tape.

You can also use black, since there's no particular need with a water heater to match up phases.  A 240V circuit with black-black hot wires is perfectly fine.
If it were in conduit, that would be a different deal. Because individual wires are easy to swap, you're required to use the correct color in the first place - no remarking is allowed.
If you're absolutely militant about refusing to mark wires, then my only advice is to use /3+gnd cable (black red white) and just cap off neutral.  That would allow you to use one of the cables for a subpanel if the tankless heater doesn't work out.