Well, technically they are not. There is a bit of debate about their genetic makeup. Some even say they are related to geese. In the food sense, as most duck that you get for food are Pekin, they are not even close. Muscovy meat is similar to beef, some say veal.
More info:
Genus Cairina
It has two species, which are similar anatomically but quite distinct in external morphology. These were initially placed as type genus in the “Cairininae” (or “Cairinini”), a supposed group of “perching ducks” which was somewhat intermediate between dabbling ducks and shelducks. However, this assemblage turned out to be paraphyletic, and the Cairina species were moved to the dabbling duck subfamily Anatinae, to which they seemed closest from the data available at that time. Analysis of the mtDNA sequences of the cytochrome b and NADH dehydrogenase subunit 2 genes (Johnson & Sorenson, 1999), meanwhile, has indicated that this is probably not correct, and that moreover the two species usually united in Cairina are not even closely related to each other, which is also suggested by the biogeography of their distribution: The Muscovy Duck seems a distant relative to the genus Aix which for example contains the North American Wood Duck. Together, they appear related to the shelducks and C. moschata would thus be placed in the Tadorninae. The White-winged Wood Duck, on the other hand – which has sometimes been allied with the enigmatic Hartlaub’s Duck (Madge & Burns, 1987) – should according to the molecular analysis moved to its old genus, Asarcornis, and could in fact be a peculiar diving duck.