I was diving in mine pit and had buddied up with another much more experienced diver who I had not met before. The dive went well and we were working our way back to the point of entry/exit in 30 feet of water when she signaled that she was going up. I didn't know this at the time, but she just wanted to see exactly where we were and thought I would just wait for her to come back down. So I slowly followed her up. I surfaced behind her and dropped my regulator and went to spin around to talk to her and she was gone. I as I did this I went under and was not ready and breathed in a huge amount of water. I did not have reg in hand so I did the arm sweep and recovered it luckily on the first try, coughed out the water. This involved no thought training took over. This stupid incident could have turned out much worse. After I got done coughing I went back down and she was right there coming back to look for me. Whole thing took about a minute. Never told her or anyone about it til now. I was too embarrassed. What I learned is you keep the reg in your mouth until you have all your s**t wired tight. Positive buoyancy established and floating comfortably on the surface and always breath cautiously. Just basic OW training material affirmed.