Transparency? Ray Tracing? You need ray tracing on to caculate where the light would fall, and if you are getting a solid shadow, you might be just affecting the water's color, not it's transparency.

Imagine a piece of paper with a picture of water on it.. if you looked under it, the shadow would be solid, not with water caustic reflections, and light bending refractions.

Way too many people take way too many things way too seriously.