I stumbled upon this experimental process where they fill the lungs up with a special liquidized gas. It has promise for diving and medical treatment, and it occurred to me that perhaps someday in the far future this could become safe and common enough to fill a tank with this 'water' and with the correct procedures 'breath underwater' and actually breathe water like a mermaid. I know the odds of this happening in my life time are slim to none the possibility excites me and causes me to wish it to happen now rather then later.

Wiki explanation.