Experience of Diving

I went half a way of other side. It has two aspect; world of under the sea and dead. I was swimming with air bottle on my rear. So I think I could breath in the sea, but not! Instructor said, "You can breath as long as you put this into your mouth. Don't put it out, or you would drown. Ok. Let's go!" But I can't breath!! I frightened so much. I feel I would be dead if I went to the deep sea this condition. So, I said "...I'm waiting here." After that I can breath in the sea. Then we landed. 20 minutes had gone very quickly. I gurked, burped, and belched.