Corals are endangered from climate change, due to exposure to sunlight during times of especially high water temperature. However, constantly shading corals also harms them. We thus introduce ReefSense: a passive computational system that can help shade corals using the energy of the ocean to sense and actuate. We have prototyped and tested ReefSense setups with 3D printed-- and off-the-shelf components: the key aspect is that the system uses the energy of temperature change and water flow to deploy shade when it is needed. We discuss ReefSense, a bachelor thesis project, as a point in a more general space of computation that can work with nature instead of against it, and allow future computational systems to integrate more tightly with the world.