Main Question or Discussion Point
Could a small amount of water be made somehow to create enough electric current to make a small heater bar hot or to even warm up?
You forgot. Neils Bohr had one final technique when asked an analogous question about the barometer.Yes. There are several ways it can be done. By "small amount of water" I will suppose you mean 1 gallon since you did specify.
1. Build a cart with a generator attached to 1 or more wheels. Place the cart at the top of a large hill. place your jug of water on the cart. Convert the gravitational potential energy of the water into electricity, and then heat your bar.
2. If the water in question is hotter then some other mass you have access to you could build a heat engine to capture energy moving from the hot water to the cold mass and convert it to electricity. This will also work if you have access to a mass that is hotter then the water, you just need to turn the heat engine around.
3. You could place the water in the core of a star. The heat and pressure will cause the hydrogen and oxygen to undergo nuclear fusion. Place solar panels around the star to capture the energy and turn it into electricity.
4. Place the water in the path of a planet or spacecraft with a high relative velocity. Instruct the inhabitants of the planet or spacecraft to construct an apparatus to convert the energy of impact into electrical energy.
Ok so there are means of extracting gravitational, thermal, nuclear, and kinetic energy from water. I'm sure there are more kinds of energy then that in a gallon of water but that's what comes to mind right now.