SCUBA diving is an activity in which divers remain underwater for prolonged periods by using a self-contained breathing apparatus. Diving is safety critical because changing depth too rapidly or running out of oxygen before surfacing can result in life-threatening consequences. These risks are currently minimized by using a wrist-mounted, 'air-integrated' dive computer that monitors time, depth and air tank pressure (received through expensive wireless transceivers or a hose). These computers are costly for the average recreational dive. We present a hybrid systems model and safety proof for a SCUBA diving computer that estimates air consumption of the diver using commodity heart rate sensors, instead. We employ a mathematical model of oxygen uptake in response to exercise and thereby predict the time remaining for the air supply to be depleted, as well as obviate the need for a tank-mounted wireless transmitter or the cumbersome hose integrated computer for directly monitoring tank pressure. We formally verify a controller that ensures the diver can always surface without running out of breathable air. c 2019 Copyright held by the owner/author(s). Publication rights licensed to.