wait, just found it! Damn, 2 days searching and when i finally give up there it is. It's Lost for love by Queen MiMi of Wonderland.
turns it it was actually a Twin-who-lived Tom/Harry fanfic - i remembered this one much better than i thought, just didn't remember that this scene belonged to it. it is unfortunately incomplete.
This is most likely La persistencia de la memoria by ScorpioPhoenix
Harry loses 5 years of his memory and doesn't remember his
relationship with Draco. Who ended up pregnant by a strange curse.
Harry tries to re-live his past, as Draco fights to keep his present.
But will their love win in the future? Rated: Fiction M - English -
Angst/Romance - ...
It's close enough for me to put forward as a potential answer, perhaps Pursuit by quinault?
“What are you?” Hermione hears herself asking, hating the slight tremble in her voice as she says it. She knows now she should not have wondered, she should not have leaned over the stern, her eyes following his pale body, the magnetic pull of his dark eyes, the ...