If water is transparent, why can we see its shadow?

Solar radiation passing through water decreases in intensity exponentially due to scattering and absorption caused by substances in the water, which reduce its transparency.

In distilled water at one meter depth, almost 100 percent of the short-wave (blue) radiation arrives, while only 40% of the longer-wave (red) radiation arrives. Overall, only 50 percent of the amount of incident light “survives” there.

The simplest instrument for measuring the transparency of water in a lake is the Secchi disk, a white disk 30 cm in diameter attached to a rope.
The Secchi disk is submerged in water and the depth at which it becomes invisible from the surface gets measured. The transparency of a lake is thus defined as the depth at which the Secchi disk disappears.

For example, it is about 40 meters in Lake Baikal. At our latitudes, the transparency of a lake is highest in the period of lowest plankton production (winter) and lowest in summer-autumn.

This is how the transparency assessed with the Secchi Disk varied in 2002 in Lake Maggiore, italy: