arusr:
Ok, I see I missed a few details:
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The bread-board Arduino is battery powered. I was hoping the input would go high (or low) and act as an interrupt to wake up from sleep. So my preference is for a digital input. Then I'd program in the nRF24L01 to send out a couple of pings to a gateway, which will then send an email alert.
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That sends you right in the arms of Nick
Make the processor sleep, only to wake up once in a while and measure the state of the sensor.
If the sensor is pulled low by water, you can initiate the alarm with emails, SMS, sirens and a flashing strobe light.