If the original thermostat is a mechanical points type, it should work fine switching the 48 volts DC at under 1 amp to drive a 48 volt DC contactor to switch the current to the heating element. You will see many relays have a 120 volt AC rating and 48 volt DC ratings at a lower current. Be sure to have a clamp diode on the contactor coil as well. That will also keep the opening voltage spike down.
If the factory element is 120 volt, you can use it on 48 volt DC, but it will be about 1/4 the watts heating the water. It will take longer to recover, but it will still get hot.
50 VDC switch rating is probably fine at low current, but most 48 volt battery systems are over 55 volts most of the time. Mine charges to 57. I like to see 60 volt ratings.