How to measure
You can use a water gauge attached to a solid structure, or you can use something like a secchi disk and note when the line goes slack. Please be aware that the location you take depth readings from must be conistent! Docks should be anchored and remain in the same spot year round. If you take your dock out every year, your readings will not be as accurate. Note this in your location description.
How to interpret:
Water depth naturally fluctuates throughout the season. Tracking water depth at times like 'highest' in the spring and 'lowest' in the late summer can give you indications of exceptional drought or flood years. Every area has different fluctations and 'normal' values. Over time, you will begin to understand what is unusual for your body of water.