This depends on the turbidity of the water. In very clear water exposed to direct sunlight, up to 18 in. of water may be needed to prevent the sun from affecting the readings. Less sunlight penetrates more turbid water, which means the sensor would not need to be positioned as deeply. If the target water is clear and shallow, it may be necessary to construct a canopy above the water to shadow the monitoring site.
In addition, the sensor must be positioned in the water so that nothing obstructs its field of view. In very clear water, this may require an 18 in. standoff.