Hello: About that electric shower 😯 I would use #6 copper wire to bond hot and cold together above the heater, then take that line and run it to the ground bar in your electric panel. This should give electricity an easier path to take than through you to get back to ground. 😎
ps. I was thinking AC… For a DC powered heater, I agree it’s safer, but you’re still running a voltage that could do damage, so how about running that #6 wire to a good grounding rod?