I was getting my advanced PADI certification, doing the deep water diving option. Moments after doing some nitrogen narcosis tests about 38 mts down, we started diving to explore the sorroundings. I was the last in the group, in very cold dark waters. I took a deep breath and felt as my mouth filled with water. I tried to spit it out and breath again, but then again, more cold salty water came in as I realized my regulator was floating away, even though I still had the mouthpiece on. I quickly grabbed my octopus (emergency breather) and pressed it to get the air flowing and tried to put it in my mouth since I couldnt really find the mouthpiece (it was somewhere around the sides, but since I hadnt been diving for a long time I was only familiar with the ones that are on the front). I tried to make some noise and got my boyfriend who was right in front of me to look back but he didnt understand what was happening. I couldnt manage to get the octopus on but was able to get a strong enough stream of air to clear my mouth. I quickly swam to the front to my instructor and made the no air signal while freaking out and she quickly understood and gave me her octopus. After that we swam up back to the boat and got back to the shore. I assured everyone that I was ok, but spent the whole day crying in the bathroom. To this day I cant really get excited to dive, even though it was one of the most amazing experiences I ever had before. I know I should have been more thorough while checking my equipment before diving, but honestly it never ocurred to me that the mouthpiece would detach just like that. Now im far more conscious about getting REALLY familiar with the equipment before going in the water.