The line is a dedicated line from breaker with 15Amp breaker...running a computer. I want to pull a line off with GFCI outler for a Rinnai Tankless water heater. I am told the WH only pulls about 70w. Can I do this and what is the proper wiring diagram, please?
The existing outlet probably has another set of screw contacts specifically for a downstream outlet. If it does not, you should replace it with a newer outlet.
Make sure you hook up the ground to the new GFCI.
I'm gonna guess that if you ask the manufacturer or an electrician they will suggest or require that you add a new run from your panel with a dedicated breaker. But as a DIY'er, I would run it as you said from an existing run.