Some help with buzzer schematic and reducing power troughput
you need to stop making bad assumptions and define what you must have/use, the - float switch and any power source, not just the 24V "bell" {low VA} transformer   
Define any important feature , like connect to a power relay to disable burning out the tank water heater? Beep ? instead of steady and have a self test?


Is it SPST-NO or NC?  What is the current rating? Is it gold-plated (&lt;2A) or non-corrosive?


small Buzzers come in 2 common types.
Magnetic 10~100mA  3~24Vdc
Piezo  1~10mA  3~24Vdc with built in oscillators
Others range from 1.5Vdc to 220Vac  


These are estimates. DigiKey et al will show what is in stock (hundreds of types)

So you need very little power P=V*I  and varying resistance with fairly low current  I=V/R

You can often waste power with a series voltage drop resistor if it is too loud, or put tape over the acoustic hole to reduce 100 dB to 85 dB when looking at rated current and loudness.