basic idea is to have current flow through water that is touching an exposed wire. connected water would then behave like lava, damaging creeps and players, no fire ignition though. this would cause a constant drain on stored power and fry any power consuming devices (non genorators). The result would be the need for more thoughtful building and some easy electrocution traps.
an more fantastical addition could be some kind of power stablizer that could turn the erratic current flowing through the water back into usable current turning water into semi-effective no-cost wiring.