Here is a site with a list of 17 species of diving ducks. Click on one of the species in the left coulumn for a photo and biological info (there are three types of mergansers listed). And click on the "Swans and Geese" link, Dub-Dub, for the answer to your question about their difference from ducks (there's also a "brant" link there):

http://lavoieverte.qc.ec.gc.ca/faune/guide/html/canards_plong_e.html