Identify this waterfowl species?
I am not an ornithologist, but using identify.whatbird.com and a simple google search, this appears to be a Muscovy Duck. Try a google image search for yourself! Some of the birds pictured have more ostentatiously red or larger caruncles, but some look very similar to the birds you photographed. For example: http://www.birdinginformation.com/birds/ducks/muskovy-duck/

From muscovyduckcentral.com and identify.whatbird.com:
The domestic breeds can come in a variety of colors, but are primarily black and white with red and white bills. The males are larger than the females -- around 15 lbs for a male and only 8 lbs for a female. Females are duller colored than the males. Muscovy ducks are indigenous to Mexico, Central and South America, but are now raised in many different locations. They are considered a good duck stock because they are large, breed quickly, provide good pest control, and are quieter than many other duck breeds.