Dennis began scuba diving during the mid-1970s as part of a research project. At the time he was a research associate at the Arizona-Sonora Desert Museum in Tucson, Arizona, studying the population of winter hibernating sea turtles. What began as a scientific study soon became a conservation project that expanded to three species of sea turtles along the entire Pacific coast of Mexico. This project received major funding from the World Wildlife Fund and was eventually taken over directly by that agency with Kim Clifton and Dennis Cornejo as co-principal investigators.
After concluding his research on sea turtles in the mid-1980s, Dennis enjoyed recreational diving while pursuing various terrestrial research programs. Among them: the feasibility of using desert legume trees for dry lands agriculture with Richard Felger at the Arizona-Sonora Desert Museum; the community ecology of desert toads and their tadpoles for a master’s degree in ecology and evolutionary biology at the University of Arizona, at Tucson; and the morphology and biogeography of giant North American cacti for a doctorate in botany at the University of Texas, at Austin.
In the mid-1990s, Dennis became a diving professional, qualifying as a NAUI Instructor, and has now dived around the world. He has also worked as a guide, instructor, and dive boat captain in the Monterey, California area. Dennis has worked with Lindblad Expeditions-National Geographic since 1981 exploring more places by foot, by sea, and under the sea, than he can remember.