What is the origin of 'aqualung'?

Without evidence, I think it was invented by Ian Anderson.

I don't see it so much as simply a homeless person but as a homeless person with some kind of horrible deep wet cough, "spitting up pieces of his broken luck". Hence, aqua (water) + lung.

This was invented by Ian Anderson:

From the linked-to article:

The idea came about from a photograph my wife at the time took of a tramp in London. I had feelings of guilt about the homeless, as well as fear and insecurity with people like that who seem a little scary. And I suppose all of that was combined with a slightly romanticized picture of the person who is homeless but yet a free spirit, who either won’t or can’t join in society’s prescribed formats. So from that photograph and those sentiments, I began writing the words to Aqualung.”

Anderson, who imagined homeless men would have breathing problems, named the title character after the famous brand of undersea breathing gear used by divers.