Low-power water level alarm
A few simple mods are all that is required. 


Add a p channel MOSFET to act as a ON/OFF switch for the 555. The circuit only draws current when the switch is activated.

The current through the MOSFET is small so anything that can carry 500 mA or more would be more than capable.

I've also simplified the oscillator to run from a single resistor (50% duty).