The singer was with her instructor at a lake near Heathrow Airport when she replaced her mouthpiece incorrectly and breathed a lungful of cold water.
Melua, 25, wrote on her official blog that she felt like an "idiot" for making the mistake and thanked her instructor for helping her to safety.
She wrote: "When I tried to replace my regulator I didn't do it properly, so I breathed in and choked on a lungful of water. I tried to stay calm and clear the regulator but I kept choking on water. Because of the pressure, the force and speed at which the water invaded my mouth was such a shock. I did start to panic, especially as you can't go to the surface because of the bends."
Melua said it was the first time in about 40 dives that she has had to make the 'out of air' hand signal. "Luckily my teacher was right there helping me through my sorry state of horror. Looking back at it now, I was such an idiot," she said.
Melua, who is working towards a drysuit diver qualification to fulfil her ambition of diving in the Arctic or Antarctic Circle, added that she hoped the experience would not dissuade fans from trying the activity.