My first real bondage fantasy was also at the age of 10, after reading a book, which I later found out was written by L.J. smith, where I am female, in a swimsuit, standing in the middle of a room.
Then a woman walks up to me, and fits on a straitjacket and scuba mouthpiece in and adds on the goggles, while I am being bound and weighted along my body.
Finally i am put in a huge steel coffin, which is then filled with water, and my body floats in the center of the tank.
I am left there infinitely to go insane on my own, with no chance of freedom ever.

I didn't know much about the biology of it at the time, so it was rather simplistic. Sadly, the book is out of print now, although it was probably way too old for me at that point in time (or so my parents would have thought lol)
Badspyro