Ace reporter Valerie Hudson (Mara Corday) and her friends discover a submerged body while scuba diving off the California Coast. When U.S. Bills are found on the victim that match the serial numbers of those that supposedly went down on a sunken ship, Hefty police Lt. Mike Travis (Dan Seymour) jumps on the case. When the Navy catches wind of the situation they bring in their own man, Lt. Brad Chase (Pat Conway), who as it happens is Hudson's ex-boyfriend. When our trio traces the bills back to the crew of a tuna fishing vessel the scene is set for a sub-aquatic crime-fest.
Though the movie has sort a of a cool period piece vibe it fails to create much energy. Despite the title Corday doesn't spend a lot of time under the sea. Beyond finding the aforementioned body and another junket where she finds herself in peril during the obligatory shark encounter, she rides out the movie on dry ground. The forced injection of a battle-of-the sexes match-up between Corday as the tough woman reporter demanding respect and Conway as the glib Navy lieutenant wears thin quickly and doesn't provide much lift. The movie hints at different sub-plots and situations which ultimately stall out. Superfluous lounge singing number launched midway through the movie adds little more than time killing filler.
Undersea Girl has no pretense about what it is, a low budget B double biller. Even so Nacirema Productions doesn't come close to clearing the bar that they set for themselves with their previous film, the similarly named Hot Rod Girl. Instead this looks like an attempt to springboard off the relative success of that film. While certainly no masterpiece, Hot Rod Girl had a sense of fun about it which is absent here. Other than the location shots and some okay underwater photography there isn't much to recommend.