The first problem you have is that throwing a toaster into a bathtub in Hollywood somehow turns all of the water into this mysterious substance called “electrified Hollywood water” which is instantly fatal. That’s not how it works in the real world. So right from the start, your premise is flawed. You can’t reliably murder someone by tossing a toaster into the bathtub.
This doesn’t mean that it’s safe to throw a toaster into a bathtub, though.
Electricity flows in a circle if sorts, which is why we call it a circuit. It goes out through the hot wire, through whatever electrical load is present, and back through the return wire. In residential electric service, we use alternating current, which means that 60 times every second (50 in other parts of the world) the electricity stops and reverses direction, making a sine wave if you were to graph it. So half of the time, it goes out through the hot wire and back through the return wire, and half of the time it goes out through the return wire and back through the hot wire. So why do we have a “hot” and a “return” if they are both basically the same? That has to do with grounding, which has everything to do with your bathtub scenario.
You can run AC electrical systems that aren’t grounded. These are called isolated ground systems, or more commonly, just are called isolated systems. If you are unfortunate enough to be in the hospital, look for the red colored outlets. Those are isolated. Isolated systems are safer in that you can touch either wire and touch ground and not get shocked. So the next obvious question is, if it’s safer, why don’t we run all electrical systems that way? The answer is that if you try to run an entire residential electrical service that way, mother nature will randomly ground your system with tree branches and such. It is very difficult to maintain an isolated system, and we would much rather have a ground system that we make intentionally than a random ground system caused by mother nature.
So this is why your toaster is a shock hazard. One of the electrical wires (the return, aka the neutral) is physically connected to earth ground. Your water pipes, being metal and conductive (if you don’t have pvc) also go into the ground. So basically, all of your water and drain pipes are electrically connected to the neutral wire in your electrical service. Drop a toaster into the tub, and now there are two ground paths that the electricity can take. One is back through the toaster, the way it was intended to work, and the other is through the water, and down through your water pipes into ground.
Because of the shock hazard, all modern homes are required to have ground fault interrupters (GFI or GFCI for ground fault circuit interrupter) in any “wet” locations, like bathrooms, kitchens, etc. You toss your handy-dandy toaster into the tub, the GFCI detects that the current going out through the wire doesn’t match the current coming back in, and the outlet pops off, leaving your intended murder victim to glare at you rather angrily and forcing you to use something a bit more reliable, like holding their head under the water to drown them.
But, for the sake of argument, let’s assume that this is an older house, built before the days when GFCI outlets were required in the bathrooms. Now you toss your toaster into the water, and part of the current goes back through the toaster and through the wire, and part of it goes through the water, and through your victim, and down through the water pipes to ground. At this point the natural tendency is either to jump up out of the water or to grab the toaster and throw it out of the tub. The latter choice is very, very bad for you, as this results in part of the current going through the toaster, and part of it going down your arm, through your chest (that’s the really bad part), through the rest of your body, into the water, and down the pipes, etc.
But here’s the thing. Relatively low level shocks like this are not guaranteed to kill. The risk here is that the electricity will interfere with your heartbeat, and this is a bit hit-or-miss. The shock has a decent chance of throwing your heart into fibrillation. Your heart has a funny design in which this fibrillation state is stable, meaning that the heart will stay in fibrillation quite happily until something else forces it out of this state (like a portable defibrillator). While it is in fibrillation, your heart isn’t pumping blood. It’s kinda just shaking chaotically. You pass out in a matter of seconds, and if no one does anything, you die in a few minutes from the lack of blood and oxygen to the brain.
Fibrillation isn’t guaranteed, though.
And, if your victim jumps out of the water instead of grabbing the toaster, more current flows through the water than through them, and the chances of fibrillation occurring are much, much lower.
So a bathtub alone is actually quite survivable.
On the other hand, under two identical sets of circumstances, one person might go into fibrillation and the other might not (there’s a lot of luck involved and the heart is more susceptible to disruptions at certain parts of its cycle than others, so exactly where the heart happens to be when the shock is applied also matters). So this is definitely not a safe thing to do, and there’s a really good reason that we invented GFCIs and have made them mandatory in bathrooms.
I’ve never tried it myself, but I’ve heard from people who have had electrical faults in flooded basements and swimming pools. They describe feeling a tingling as they waded through the water, starting at about maybe four to six feet away from the fault. Note that if you can feel the tingling, there’s more than enough current there to cause fibrillation, so this is definitely one of those “don’t try this at home” types of things. The tingling gets more intense as you get closer to the electrical fault. Again, it’s not a safe thing to do, but in Hollywood there would be sparks everywhere and everyone who was even partially touching th water from any distance would die very dramatically.
You can wade through a basement or a pool with an electrical fault, and probably won’t be killed. A bathtub is kinda iffy. Chances are you wouldn’t be killed, but I certainly wouldn’t want to risk it myself.