UPDATE: A new version is here!
It's fundamentally the same device - a coin cell with a flashing LED, and a switch connects power when water is low. But I have improved the sensor part and used a nice PCB.
The sensor is a sealed unit that is mostly an expanded foam floater, with a tilt sensor inside. The sensor acts as a switch. The whole floater assembly is waterproof-sealed. From Sparkfun's description of the tilt sensor:
"Inside the can are a pair of balls that make contact with the pins when the case is upright. Tilt the case over and the balls don’t touch, thus not making a connection."
The floater is anchored on one side so that when floating in a full tank the tilt sensor will not trip. But when the water level is low, the sensor "hangs" from the wire and the tilt sensor trips.
Here I laser cut the top out of black acrylic:
And the finished product! When the battery need changing (which hasn't happened yet!) or for some reason we need to get to the PCB, just lift off the top.
I wonder if these might make a good kit. I can't be the only one who finds managing the water level of the machine to be troublesome!