MOSFETs leaks current between two water sensors
looking at everything so far, it appears to be an EMI with total disregard for stray line noise on high impedance sensors with water acting as a large capacitive antenna for line noise.

Use all the usual methods of shielded twisted pairs , and check signals on scope or DMM for DC and AC using your good central ground.  Active guarding may be the best solution, which you can look up.  But first measure/verify input signals.