Decal Projectors cannot affect the water surface, because it is transparent in nature and therefor doesn’t write into the render pipeline’s depth texture.

Instead, this asset features it own – water specific – decal method.

> You can find several premade decals in the Prefabs/Effects/Decal folder

Water decals are regular plane meshes, using a specialized shader. This shader reads out the water’s height at every vertex position, and then snaps its Y-position to that height. As a result the plane appears to float on the water surface. Effects such as waves and dynamic effects influence them as well.