Urban greening -- planting trees, shrubs, and grasses -- is widely promoted to reduce extreme heat in cities. However, in arid regions, the effectiveness of this cooling and the amount of water it requires remain uncertain. In this study, we investigate how water availability and climate conditions influence the ability of vegetation to cool the environment in and around San Diego, California. We use publicly available data and data-driven analyses to explore how vegetation interacts with its environment to produce cooling and how much water is needed in the process. We find that shrubs and grasses may cool more efficiently than trees in areas with lower baseline vegetation, while also using less water. In contrast, trees are more effective in greener areas but require more water. We also find that substantial increases in vegetation are needed to achieve temperature reductions reliably, and that vegetation cooling may depend on soil water availability. These findings can inform urban planning strategies that balance cooling benefits with water demands in heat-prone, water-scarce regions facing the growing impacts of climate change.