We present a theoretical model for calculating the locomotion cost of breath-hold divers. Starting from basic principles of mechanics, we calculate the work that the diver has to provide with propulsion for counterbalance the action of the drag, the buoyant force and the weight during the immersion. The basal metabolic rate and the efficiency to transform chemical energy in propulsion are also considered for the calculation of the total energy cost of a dive. The dependency on the diver and dive characteristics and possible optimisations are analysed and discussed. Our results are compared to observation on different breath-hold diving animals. The model confirms the good adaptation of dolphin for deep dives, and it gives some insights for a possible explanation of the exhalation of air before diving observed in seals. A comparison between predicted and observed swim velocities of different breath-hold mammals confirms the importance of the role of the diving reflex.