its 220 volt, 20 amp.

google NEMA 6-20R

you could buy a NEMA 6-20P plug at a hardware store and just cut the end of the power supply cord and put it on. ONLY if you are SURE youll do it correctly of course. no sense dying over it..

where are you? Ive never hear of a whole house hot water heater with a plug. all that Ive seen (hundreds in my prev job in construction) were hard wired 220 volt.