In light of the presence of Legionella in springs and of the antibacterial activity of cinnamon oil, we conducted this study to determine the bioactivity of cinnamon oil against L. pneumophila at various pH and in hot spring waters typical in Taiwan. The culturability of L. pneumophila at pH 2–10 and in hot springs was also assessed in order to characterize the risk of Legionella exposure during bathing activities. Moreover, the effects of dilution with tap water on cell culturability and on inactivation efficacy of cinnamon oil in spring waters were also investigated as this practice of blending is commonly conducted in spring bathing to lower water temperature.