Hello, it should be possible by measuring the resistivity of the water layer. This should correlate with the thickness of your sample. You only need 2 electrodes on your sample and some measuring device. Perhaps you can also use a quartz crystal. They change resonant frequency with the thickness of teir coating. But i dont know if this works for liquids. A third option works on conducting substrates. You can simple use a needle where you can adjust z position in µm range. When the needle touches the water courent flows that can be measured. The same principe like a tunneling microscope. You can also freeze the water and use a profilometer to measure thickness. This device has nm resulution.

Hello.

You can use optical interferometter method or try measure using cheap laser's beam deviation  as a result of beam refraction..

You also can use ellipsometer for thi s purpose..