What should be done if a diver convulses underwater?

When a diver convulses underwater, holding the regulator in the diver's mouth is crucial for their safety and survival. If a diver is experiencing a seizure, maintaining the regulator in position can help ensure that they continue to receive the air they need to breathe. During a seizure, the diver’s ability to control their airway is compromised, which increases the risk of drowning if they are not able to breathe effectively.

It’s essential to keep the regulator in place to prevent water from entering their lungs while also providing them with a source of oxygen. This action is part of a first aid response that divers should be trained to perform, as it helps manage the immediate risk until the situation can be stabilized.

Other options, such as leaving the diver to regain consciousness or descending to a deeper level, would not provide immediate support and could potentially worsen the situation. Signaling for assistance before checking on the diver may delay critical action needed to ensure their safety. Therefore, holding the regulator in the diver's mouth is the most effective and immediate measure to aid a diver who is convulsing underwater.