Is this a GAS or electric tankless water heater? If GAS, then I believe the outside receptacle could be used, BUT there may well be a building code requiring a dedicated source. If you have an ELECTRIC tankless water heater I cannot imagine that you would get sufficient power from a 15amp 120 volt branch circuit. All of the ELECTRIC units I have seen were 240 volt rated, and 20 amps or higher.
In the case, I would make a trip or phone call to the sumter county building department ask for the electrical inspector, and then ask him, this will get you the correct answer, but you will need the information I asked above.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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