Hanzo couldn’t help but wonder at the man, as he slept. It was always the most peaceful he saw him, and made the mer feel content. Gently he touched the human’s lips, they were dry, chapped. Just as he had suspected. But how he wished he could kiss them. 

So in this AU if a merman/mermaid/mer kisses a human, they turn human and can never go back until the human dies. (which sucks since Mers are basically immortal) That’s just how it is. Hanzo would like to, but is worried that McCree may tire of him. That and becoming human is quite painful, he hears.