Hydrotherosaurus was formally named in 1943 by paleontologist Samuel P. Welles. The generic name means "fisherman lizard", while the species epithet honors Annie Montague Alexander, wealthy benefactor of the University of California Museum of Paleontology. A rancher named Frank C. Paiva discovered the first scientifically documented Hydrotherosaurus specimen in the Panoche Hills of California. Paiva's discovery was a nearly complete skeleton preserved in a Late Cretaceous rock unit called the Moreno Formation. The specimen, now known as UCMP-33912, is one of the most complete plesiosaur skeletons ever found. Hydrotherosaurus alexandrae was about 23 feet long in life. Roughly fifteen feet of this length was taken by the neck and its roughly 18 inch skull. The teeth lining both its jaws were long, hsarp, and curved. In his initial description of the creature, Welles speculated that it lived an extremely active lifestyle.
Casts of the Hydrotherosaurus alexandrae type specimen are on display at a varity of locations. Within California they can be seen at the California Academy of Sciences in San Fransisco, the University of California's McLane Hall, and the University of California at Berkeley's Life Sciences Building. A cast is also on display in the W. M. Keck Museum of the University of Nevada's Mackay School of Mines building.