This image shows the visual importance of caustics when rendering water. The same water surface is rendered without caustics (on the left) and with real-time caustics (on the right). Caustics not only improve the visual quality, but also makes it easier to perceive the shape of the water surface.

Abstract
Caustics are crucial in water rendering, yet they are often neglected in real-time applications due to the demanding computational requirements of the general purpose caustics computation methods. In this paper we present a two-pass algorithm for caustics computation that is extremely fast and produces high quality results. Our algorithm is targeted for commonly used height field representations of water and a planar caustic-receiving surface. The underlying theory of our approach is presented along with implementation details and pseudo codes.

Cem Yuksel and John Keyser Fast Real-time Caustics from Height Fields Proceedings of Computer Graphics International (CGI'09), 2009 [Preprint PDF] [Slides] [BibTeX]