when we lived in gatineau, we had a really great in-ground pool. i would put on my mask, and swim down to the bottom. there, i would lie on my back looking up at the surface. the bubbles i blew would float up in rings, and i would watch them until i was so out of breath i risked passing out.
I always really liked having my head underwater as a kid. I think it was the feeling of the water pressure and the muffling of sound that I enjoyed.