looks like a mallard duck to me! my cat brought a 3 day old one in and i reared him till he was 15 weeks old and he looked just like that! I took him to a wildlife centre to live with other ducks though because he imprinted me and thought i was mum and wasn't doing duck things lol. Heres a pic of him when he was 2 weeks old and a picture of a mallard duck you can compare yours with!
He used to have his own box with a litter tray inside for him to swim in and a stone in the middle so he had something to climb onto, a box with mud and grass in he used to have so much fun in it! Also he loved meal worms
Good Luck and have fun with your duck!!