Which of the following is a characteristic of semi-closed circuit rebreathers?

Semi-closed circuit rebreathers are designed to recirculate exhaled breath, which means that they take the diver's exhaled gases, remove carbon dioxide, and add oxygen as necessary before allowing the diver to breathe the mixture again. This system conserves gas and can extend the dive duration compared to traditional open-circuit systems. The recirculation process is efficient because it minimizes the loss of unutilized gases, making it suitable for deeper dives where gas management is crucial.

In contrast, other characteristics mentioned in the options do not accurately describe semi-closed circuit rebreathers. For instance, they do not release gas bubbles continuously; rather, the design aims to minimize bubble formation, making the diver less detectable to marine life. Additionally, using rebreathers typically requires specialized training due to the complexity of their operation and the management of potential issues like gas poisoning or equipment malfunction. Lastly, they are not restricted to shallow waters; in fact, semi-closed circuit rebreathers can be used effectively in various environments, including deeper dives, provided the diver has the appropriate training and equipment set up.