I just submitted an article on this topic for the next issue of Cornish Studies, so I have some fresh research on the topic. To answer your second question, although there are mermen, there is little question that the majority of interest in these supernatural beings of the sea focus on the females. This is perhaps because ships were traditionally "manned" by men, and the idea of enticing supernatural females luring men to their abode seemed a natural consequence of the segregated workplace of the sea. Oddly, underground mine spirits were generally male: see my article “Knockers, Knackers, and Ghosts: Immigrant Folklore in the Western Mines,” Western Folklore Quarterly 51:2 (April 1992), but these spirits were co-workers (and therefore male) as opposed to residents of the sea.

In Northern Europe, there are two species of seafolk: in Scotland, western Scandinavia, and rarely in Northern Ireland, they are silkies: seafolk with seal skins. In Ireland, the rest of Britain, and in other areas of Northern Europe, they are half fish, half human.

Here are some excerpted paragraphs from my article, which may help:

The belief in sea-dwelling supernatural beings is ancient, and over the years, a number of scholars have examined the history of the motif. In 1880, Llewellynn Jewitt took a position that far-flung examples of half-fish, half-human creatures were linked in some way, and this idea was repeated, to a certain extent, by Arthur Waugh in 1960. Llewellynn Jewitt, “The Mermaid of Legend and of Art,” The Art Journal (1875-1887) New Series, 6 (1880) 117-20, 170-2, 230-3; Arthur Waugh “The Folklore of the Merfolk,” Folklore, 71:2 (June 1960) 73-84.

Although fish-human deities appear in sources from ancient Mesopotamia, the more removed the examples are from nineteenth-century Northern Europe, the less convincing the connection. In 1990, the late Bo Almqvist took up the motif of the mermaid in his analysis of a legend that inspired in two twentieth-century Irish poems.... His overview of the belief in mermaids provides a good foundation. Almqvist begins with the acknowledgement that there were “once all but universal” beliefs that people could transform into animals and that “certain zoomorphic or semi-zoomorphic beings – whether expressly stated to be enchanted humans or not – are able to remove their animal coats and take human shape.” Bo Almqvist, “Of Mermaids and Marriages: Seamus Heaney’s ‘Maighdean Mara’ and Nula Ní Dhomhnaill’s ‘an Mhaighdean Mhara’ in the Light of Folk Tradition,” Béaloideas 58 (1990) 1. See also Dáithí Ó hÓgáin, The Lore of Ireland: An Encyclopedia of Myth, Legend and Romance (Cork, 2006) 342-45.

Almqvist also discusses the work of Helge Holmström and his 1919 thesis on the Swan Maiden Motif: “the Swan Maiden Legend is but one of a whole complex of migratory legends relating to marriages to supernatural or supernaturally transformed female beings. Thus [Holmström] distinguishes groups dealing with marriages to fairy women (feäktenskaptyperna), another group about personified nightmares (maräktenskapstypen) and a third one about aquatic beings, mermaids or seal maidens (säläktenskapstypen).”Almqvist, “Of Mermaids and Marriages,” 2-4. In addition, Barbara Fass Leavy, In Search of the Swan Maiden: A Narrative on Folklore and Gender (New York, 1994) notes the similarity of the “Scottish” seal maiden story to that of the swan and her feather cloak, 33-37, 46-47, but her work focuses on an engendered interpretation of a man abducting a women. Carl Wilhelm von Sydow, “Folktale Studies and Philology: Some Points of View,” (in Dundes, The Study of Folklore (New Jersey, 1965) 223-224) argues that Tale Type 400, “The Swan Maidens” is ancient; he criticizes the 1919 conclusion of Helge Holmström, Studies on the Swan Maiden Motif, that the folktale originated in India. See also Arthur Thomas Hatto, “The Swan Maiden: A Folktale of North European Origin?” Bulletin of the School of Oriental and African Studies, Vol. 24, Part 2, 326-52. Hatto suggested that the distribution of the motif is possibly linked to the Arctic and sub-Arctic and tied to “the distribution and migratory breeding patterns of swans, geese, and cranes.” How the folktale is linked to, affected by, or affects the legend ML 4080 is beyond the scope of this article, but the similarity is clear.

There are ubiquitous Northern European stories about marriages between oceanic supernatural beings and men... Indeed, Almqvist suggests that one of the more widespread legends of a mermaid entrapped by a man, with whom she is forced to live for several years, is found in Gaelic-speaking areas and in Scandinavia, but that it is not found in Wales or Brittany.... Almqvist, “Of Mermaids and Marriages,” 8, agrees with previous scholars that the legend of the marriage to the mermaid, ML 4080, probably diffused during the Viking Age from either Ireland or Scotland “directly or via Orkney and Shetland mainly to Iceland and the Faroe Islands, where influences from Irish and Scottish-Gaelic traditions are on the whole stronger than in the other Nordic Countries.” He points out that previous scholars including Liestøl and Matras (n. 16) referred to the source as “Celtic,” but as Almqvist indicates, “it would have been more correct if they had referred to the legend as Irish and Scottish Gaelic rather than ‘Celtic’, since it is not found in Wales and Brittany.”

The archetypal mermaid, with fishtail, beautiful form and face, and lavish golden hair, is at the core of the Northern European tradition, and this is precisely what one finds in nineteenth-century Cornish belief. Given the strength of the mermaid tradition in Cornwall, it is not unreasonable to see it as an important source for the image of the sea-going supernatural being in the English-speaking world. That is not to dismiss other inspirations: Hans Christian Andersen’s 1837 Danish literary story, “The Little Mermaid” – and especially its 1989 film incarnation courtesy of Disney – is arguably the most well-known contemporary expression of this type of supernatural being. Even though Andersen’s artistic invention adapted folklore in a distant way, his work has seeped back into popular culture. Nevertheless, his influential tale stands in opposition to the publications of the Brothers Grimm, as these German folklorists sought to treat folklore more faithfully while creating a body of literature for popular consumption.