Sensing circuit not working as planned with op amps
You have enough input bias current (5/5MΩ = 1uA) for it to work. The problem is the other unseen elements of the circuit. If this is a tank, you have many other common mode noise sources that could ruin your measurement (AC mains, RF). It won't take much for a few uA's to get into the water and conduct into your sensor and out through other points in the circuit. (A ground loop is also created by connecting the circuit to the water). You might (long shot) be able to get this working if you isolate the circuit from the DAC and earth ground and shield the circuit from external noise. Otherwise go back to the drawing board. Do you really need the DAC if its just an on\off?