If I was the AHJ on an inspection for a water heater, I would accept a breaker padlock device manufactured by the same company that made the breaker/panelboard.
Many mobile/manufactured homes have an electric water heater & every one that i have looked at has a padlock attachment on the breaker. This makes HUD happy & so a similar installation in a dwelling should satisfy any AHJ you deal with, but it is always best to ask if you aren't sure.
You could use one of those $8 pull out air conditioner disconnects just to CYA, but IMO, you'd be wasting $8 & some time.
Few things are harder to put up with than the annoyance of a good example.