If you find yourself exhausted and breathing hard while on the bottom, you should:

When a diver finds themselves exhausted and breathing hard at the bottom, the most appropriate action is to slow down, gain control of breathing, relax, and secure themselves to a buddy or a stationary object. This approach is crucial because it helps to stabilize the diver's physical and mental state, allowing for a return to a controlled breathing pattern. Taking deep, calming breaths can help manage oxygen levels and reduce panic, which might escalate the situation.

Holding onto a buddy or a stationary object enhances safety by providing stability, allowing the diver to focus on regulating their breathing without the added concern of drifting or losing orientation underwater.

The other options, while they may seem viable in isolation, could lead to greater risk or complications. For instance, breathing 100% oxygen at depth poses significant risks due to oxygen toxicity and is not an immediate solution for exhaustion and rapid breathing. Sharing a buddy's air could create an additional layer of stress, especially if the exhaustiveness leads to anxiety, which would make breathing even harder. Lastly, holding one's breath to let carbon dioxide build up is dangerous because it can lead to shallow water blackout or other complications associated with improper breath control.