White water bird with black 'hat'
It is a domestic form of mallard. They are bigger, often white or mostly white. Try to google image domestic mallard and you will see the phenotypic variety that domestication has created.

Note the irregular and not symmetrical shape of the "hat" and read the description below from the biggest research center on birds of the world.

Extracted from:
http://www.birds.cornell.edu/crows/domducks.htm


  (...)lots of white is often involved, including all-white breeds like the popular Pekin Duck (...) Usually these white spots are not symmetrical across both sides, and that asymmetry should tip you off to think domestic influence.