With the development of ocean research, communications from the air to underwater (e.g., aircraft to submarines) are especially important. However, the existing communication methods have their own limitations and wireless communication between underwater and the air has remained an unsolved problem. In this article, we propose a photoacoustic communication based on low-cost passive relays, which is capable of not only minimizing the energy loss both in water and in the air-water interface, but also relieving the difficulty of target seeking. Thus, we can get a six order of magnitude reduction of laser energy (25 J vs. 27 mu J) using passive relays whose cost is less than US$1.00. It is also demonstrated that our photoacoustic communication can work decently under complex water situations including waving and pollution. This creationary photoacoustic application may become a practical way to communicate from the air to underwater.