Could transporting electricity using plastic water pipes be feasible?
Water, especially pure water, is a rather poor conductor of electricity. 5–50 mS/m for tap water vs. more like 6E+7 S/m for copper. That's about 10 orders of magnitude, so for the same losses as a 4 mm diameter AWG 6 wire your pipe would have to be 16,000 km in diameter, which would make plumbing somewhat inconvenient. You'd need a return conductor too. 

That said, there are times when water (including groundwater) can be used to conduct electricity for transmission, at a cost in efficiency.