How to measure water level with an LDR?

Your question should define what you mean by "LDR."

I don't know if you are talking about a Long Distance Relationship, or a Light Dependent Resistor. Let's assume the latter. If it's a wrong assumption, then the lesson learned here is you get what you ask for--and if you don't properly define what you are asking for, then don't complain when you get "bad" answers.

Anyhow, Light Dependent Resistors vary their resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance. Thus, to measure a water level with an LDR, figure out what baseline of reflected light give you what values for how much water. Put all these results into a lookup table for your microprocessor, and you can have a digital readout from an LDR that will tell you what water level you have.