Water material should not have diffuse at all. Set it to 0 black. Anyway bidir/VCM is an experimental feature not fully supported yet.
Is better to have a caustic pass instead of rendering the whole togheter.
Ages ago i did some tuto on caustics. just use one material instead of 3 if you are not interested in diffraction. https://forum.corona-renderer.com/index.php/topic,5431.msg47346.html#msg47346
Is in the tutorial section of the forum. Is a little outdated but maybe it can help you.