Electronic – Add a relay to an existing switch

You can use a logic gate MOSFET, like the FDC855, and then your circuit will look like this:

A FET will give you a high current if needed (probably not necessary, since you're powering the motor from AA cells).

Or a BJT like the BC547, and then you would make it like this:

With the 1 kΩ base resistor you'll have minimum 100 mA current for the motor. If you need more I'd recommend the FET solution.
The diode can be a 1N4148.

I highly recommend doing a lot more research before playing with 230V AC mains!

And don't hook your reed switches to a 230V water heater. Most reed switches are fairly delicate. I've never seen one that can reliably switch 230V, let alone the amperage required by a water heater.

To answer your question, you would hook up the reed switches to inputs of your Arduino (or whichever controller you will use). Then, have the Arduino output a signal to drive a relay to control the water heater.

You will likely need two relays. A low-power, low-voltage relay (or MOSFET) that can be driven by the Arduino, and another one that can power the heater. The small relay drives the large one.

Please note that the real circuit will need additional considerations, such as inductive kick diodes and perhaps an RC snubber network for the relay.

Here is an answer that talks about driving a larger load with a MOSFET. You could use the same ideas, but put your larger relay in place of the "motor"...

As far as a simpler solution, you could do it all with relays. Then you wouldn't need to learn how to program an Arduino.

Something like this:

However, this requires that your reed switch and relay are all spec'd for 230V. And as I said before, I don't think you will find a reed switch rated for 230V AC. A better alternative (safer, cheaper, and easier to find parts) would be to use something like:

Please keep in mind that these aren't complete circuits. Only guidelines to help you research.