Oh, holy HELL. Harry might appear to be the "bottom" here, but he's FAR from being "submissive" in any way, shape, or form!
.....
You'll have to forgive me... Snape's expression is making it incredibly difficult for me to formulate coherent sentences right now. He just looks so far gone, and... gnnnuuuuuuuuuuuh.
Gah. The skin tones, the light reflecting off their skin, the darkness and intensity of the scene, Harry's presence, their hair, their hands, Harry's cock rising up out of the water, that strand of hair falling across Snape's face (I swear I started to reach for my monitor to tuck it back behind his ear for him...), Harry's collar bones, the musculature...
I want to know more about what's going on here (although I have some ideas of my own); it's just intense intense intense!