Buddy breathe with a single regulator: Buddy breathing, which is sharing a single second stage between two divers, was once a standard air-sharing method, but became less and less favored as a viable option over the last 20 years. 

Alternate air sources have made buddy breathing unnecessary, along with the fact that buddy breathing is a moderately complex motor skill to perform in an emergency.If you're deeper than 12 metres/40 feet and there's no alternate air source available, buddy breathing may be an option if you and your buddy remain calm, and if you're both trained and practiced with it. Once you begin buddy breathing, you and your buddy should continue all the way to without attempting to switch to another out-of-air option. Your instructor may have you practice buddy breathing, but keep in mind that sharing air with an alternate air source is far more preferable and makes buddy breathing an unnecessary option. Make a buoyant ascent.

You're too deep for a controlled emergency swimming ascent and you're too far for your buddy to help you. You can still make it to the surface, though the situation isn't ideal. You make a buoyant emergency ascent,. just like a controlled emergency swimming ascent, except you drop your weights. You look up and exhale continuously, making the alaahhh sound into your regulator as you rise to the surface. You're going to exceed a safe ascent rate, and that has some serious risks - so use this method only when you doubt you can reach the surface any other way. 

You can flare out to create drag and help slow your ascent if you start to rise faster than necessary to reach the surface safely.After reaching the surface using any of these options, remember that you ma ,y need to inflate your BCD orally to establish positive buoyancy. Remember to discuss out-of-air emergency options with your buddy as part of planning your dive, and stay close together so you can assist each other if necessary, especially as you go deeper. Look after one another, watching your air supplies. breathing patterns, and time and depth limits. By remaining alert and monitoring each other, you can avoid air supply and other problems.