How to detect water presence with only 2 wires
Since all you have is 2 bare wires, the water simply forms a resistor between them.

You can use it as part of a resistive divider, with a fixed resistor between your analog input and +3.3V.  The resistance between the wires will change from infinite (dry) to something much lower (wet) and the reading you get from the ADC will depict that.

You will need to experiment with different resistances in order to get reasonable values, but start with 10kΩ, or 100kΩ.  If you use a very high resistance (in the order of 1MΩ or more) you may even be able to use it like a simple switch and connect it to a digital input.

Be warned, though, that this method will cause a small amount of electrolysis, causing one electrode to be eroded away and the other to gain extra material.  For this reason it can be good to connect the resistor to a GPIO pin instead of directly to +3.3V and switch the GPIO pin HIGH when you want to do a reading (to provide power) and LOW (or INPUT) when at other times (to remove power).  This will greatly increase the lifetime of your water sensor.