“In order to get electrocuted, your body has to pass an electric current, and that current has to pass through organs like heart, lungs and brain. But the thing is, your body is a fairly high resistance. Current will preferentially pass through the water, which should be relatively low resistance. In addition, the water will act as something of an insulator between the voltage source and your body. So it will kind of go around you to reach ground, which is generally the drain.
With plastic, it won’t happen at all. There will be no available path that includes your body. Currents will only have one return, and that it’s the return wire of the toaster, which is inside the toaster.
So while tossing a toaster into the water to kill someone taking a bath is an entertaining plot device, it’s not fully reliable. Better to go with the tried and true shotgun instead.”