Modern motor vehicles often have a fuel gauge AND a 'Low Level Fuel Warning Lamp'. This is because the fuel gauge is often not very accurate but a detector located at a low level part of the fuel tank will illuminate when the fuel level is low regardless of the fuel gauge reading.
The detection is done by using a Negative Temperature Coefficient resistor as a detector.
All you need is a DC supply and connect the NTC in series with a resistor (or lamp). When the NTC is cold by being in contact with the (fuel) water, the heat loss is great and the NTC cannot heat up so its resistance is high. When the (fuel)water level is low, then the NTC heats up and passes more current which is ultimately limited by the load (lamp or resistor). The usual set up in a vehicle is to use about a 2 watt globe and a 1300 ohm NTC. This is a simple arrangement but you could use a Wheatstone bridge arrangement to do the switching when the water level falls.
This kind of arrangement would keep DC power away from the water.
Another way you might do it is to use a float arrangement which blocks a light path between a LED and a photodiode. This stuff can often be scrounged from old photocopiers and the like.
Hope this helps.