Have a rod that pivots in its center and has objects of equal mass at each end, like a balanced seesaw. But make one of the objects very low density (less than water) and other high density.

Since the densities differ, water will cause the rod to rotate. But since the masses are the same, bumps will create no net torque around the pivot point and thus no rotation.

ASCII art diagram:

    F------(x)------C

    (x): pivot point
    F: float
    C: counterbalance

I'm sure there are other ways like sensing the electrical resistance of the water.

Or just let the float sensor bounce. It's underwater when it stops bouncing and is continuously in the up position.

> Or just let the float sensor bounce. It's underwater when it stops bouncing and is continuously in the up position.

Road grime, break dust, grease, grit, etc will contaminate the shaft and make it stick, potentially telling you that youre:

A) Never in water

Or

B) Always in 2.5" of water because the sensor is stuck.