Re: 230v/240v water heaters
Since the heater element is a resistive it really won't care about the frequency. And the voltages are the same within the standard tolerances. Split-phase vs hot/neutral also makes no difference, especially installing a split-phase device on a system with a hot/neutral (one leg that is "expecting" a hot connection will be zero instead).
If there are any electronic controls associated with the heater that is a separate can of worms. Probably OK, but there is a potential for frequency to be an issue.