Resistive devices operate on either AC or DC. In the early days of electrical appliances they were operated on either: Utilites might be DC (Edison) or AC (Westinghouse), windcharger/battery systems were, of course, DC, and small water generation might have been either. AC is rated using "RMS" (Root Mean Square) volts, amps, and watts, because that way the power dissipation and heat/light production in a resistive heater or lamp come out the same. Resistive element construction hasn't changed significantly since that time.
An AC resistive water heating element should work just fine on DC. Better, in fact: The heating is proportional to the RMS current and voltage, but the peak voltage it experiences with DC is the DC voltage and with AC is the cycle peak voltage, which is higher. A DC application does not experience AC's twice-the-frequency power cycling (and resulting vibration and mechanical stresses). The only added problem with DC would be increased galvanic corrosion IF the conductors get wet. But a) that should not happen and b) if it does you have other problems.
A resistive water heating element will work fine at any voltage NO HIGHER than the rated voltage. It will run cooler and last longer at lower voltage. (It will run SUBSTANTIALLY cooler, and dissipate substantially lower power, because the power dissipated in a resistance is the SQUARE of the current through the resistance. Running a 120 volt heater at 48V only burns about 1/6th the rated power.)
A resistive water heating element's resistance varies somewhat with the operating temperature of the resistance wire. So, if you assume the resistance is constant, the power you calculate for operation at lower voltages will be somewhat in error.
If you use a thermal switch to interrupt the power to a resistance water heating element, you need to use one that is rated to interrupt DC at (no less than) the operating voltage and current. DC is harder to interrupt than AC because AC periodically goes to zero as the cycle reverses, helping to extinguish the arc.
So you should have no problems using a stock resistive water heater designed for mounting in your tank as a lower-voltage DC heater / dump load, provided your thermal cutout switch is rated for DC service and you can accept the lower power drain.