If water could be converted to plasma for a propellant by electricity how much thrust would water produce? What would be the byproduct?
That's an arcjet.
Like other electrically-powered rockets, the thrust of an arcjet is quite low, but the specific impulse is good. Hydrazine, ammonia, and hydrogen are the most-often proposed propellants for arcjets, but water has been used experimentally as well (paywalled paper, so I don't actually know the thrust value they got). Specific impulse is around 500-600 seconds; thrust would be on the order of 100-200 Newton per megawatt of power input. The exhaust would be water in some form or another -- water vapor, droplets, or some combination.