We present a novel mixed reality (MR) rendering solution that illuminates and blends virtual objects into underwater 360° video with real-time underwater caustic effects. Image-based lighting is used in conjunction with underwater caustics to provide automatic ambient and high frequency underwater lighting. This ensures that the caustics and virtual objects are lit and blend into each frame of the video semi-automatically and in real-time. We provide an interactive interface with intuitive parameter controls to fine tune caustics to match with the background video.