30 ampTankless Water heater
The small tankless I have in my barn is very similar,  30 amp 2 Hot’s but the little array that actually heats the water is metals and has a place for the equipment ground. It would be a good idea for safety to connect the ground.
 Will the heater work without the ground? Most will. The heating element is 220v and all but 1 of these small units I have worked on were controlled by switches for flow no flow the switch could not close and heat the water. Don’t expect much out of this small unit it will get the water hot but how hot depends on the incoming water temp and the flow. For us it is just enough to heat the water to bathe horses before showing them.