"Is the Lorelei really a mermaid? "
Always thought she was a siren

"Silkies are not mermaids"
Technically that is true, but they are part of the whole bundle of Folklore depicting sea creatures in hunan form and the constantly cross over in the tales
The Silkie story/song of a sea creature marrying and having children by a human man/woman are so similar as not to be a coincidence and both stretch back into ancient International mythology

From Funk and Wagnall's Standard Dictionary of Folklore
Mermaid
A supernatural sea-dwelling female of general European maritime folklore: German Meerfrau, Danish maremind, Cheremissian wut-ian iider. (See HAVMAND, the Estonian NAKK, Finnish NAKKI.) The Irish mermaid, murdúac, is Anglicized to merrow. The Morgans, or sea-women, off the coast of Brittany are considered beautiful, siren-like, and dangerous to men. Mermaids are usually depicted as having the head and body of a woman to the waist, and a tapering fish body and tail instead of legs. A carving on Pucé Church in Gironde, France, however, shows a young mermaid with lower body divided and two tapering tails instead of legs. They live in an undersea world of splendor and riches, but have been known to assume human form and come ashore to markets and fairs. They often lure mariners to their destruction, and are said to gather the souls of the drowned and cage them in their domain. Those who seek fact underlying every belief have offered the manatee or the dugong, warm-blooded sea mammals, as the original for the mermaid, relying on analogy more than on sailors’ ability to differentiate between a sea- cow and a fish-woman.
Concerning the origin of mermaids, the Irish say they are old pagan women transformed to mermaid shape and banished off the earth by St. Patrick. A Livonian folktale says they are Pharaoh’s children drowned in the Red Sea (B81.1). For origin of the mermaid concept S. Baring-Gould suggests (Curious Myths of the Middle Ages, London, 1884) the various semi-fish gods and goddesses of the early religions, e.g. the Chaldean Oannes and the Philistine Dagon, especially when associated with the sun-god concept, as Oannes who appeared on earth every day and plunged into the sea every night. Atargatis (Deiceto, Dea Syria) too is sometimes depicted with semi-fish body. For association of the mermaid with the ancient sea goddess and love goddess, Marian, identified by the Greeks with Aphrodite, born of the sea, and the symbolism of the comb and the mirror, see Robert Graves, The White Goddess, New York, 1948, pp. 327- 328; for explanation of the fish tail in statues of Atargatis-Derceto, p. 335.
Medieval belief in the mermaid, however, was wide¬spread and substantiated. There is a Netherlands story reported by Baring-Gould (p. 509) that in 1430 when the dikes near Edam broke in a storm, some young girls in a boat found a mermaid floundering in shallow, muddy water. They got her into the boat, took her home, dressed her in women’s clothes; she could weave and spin with extraordinary skill, but never learned to speak. Old Henry Hudson is said to have seen one near Nova Zembla on his arctic explorations. In 1560 west of Ceylon some fishermen caught several in a net, which were taken ashore and dissected with great interest by a learned physician. He reported that internally and externally they were constructed like human beings.
It is generally thought unlucky to see a mermaid; the sight of one presages storm or disaster as witness Child’s ballad The Mermaid, in which one of a ship’s crew sighted a mermaid sitting on a rock with comb and mirror in her hand. Very shortly the ship was lost in a raging storm and for want of a life¬boat all hands were drowned. Mermaids also often lead people astray. Getting hold of the cap or belt of a mermaid gives one power over her. The marriage of a mortal man to a mermaid is common in folktale and folk belief; descendants of such a union are still living in Machaire, Ireland. Such people are usually under some kind of a curse; either they cannot sleep at night for the haunting sound of the sea in their ears, or they are doomed not to speak.
The heroine of The Little Mermaid by Hans Christian Andersen falls in love with a beautiful prince on a passing ship, voluntarily assumes human shape in order to gain an immortal soul and be near him forever, is doomed never to speak, attends the prince constantly, but when he marries a human princess her heart breaks. She becomes an elemental light being, with a chance at immortality.

Merman
The male counterpart of the mermaid in general European and Near Eastern folklore. Tv Arabian Nights refer to the merman both as human anc as fish. “The Tale of Abdullah and Abdullah” in the Arabian Nights is the story of the poor fisherman named Abdullah and his benefactor, the merman named Abdullah. Al-Kazwini refers to a Syrian story of a merman married to a human wife, whose son knew both tie language of the earth and sea. Probably the mcs familiar merman motif of folktale and ballad is the merman forsaken by his human wife, widów popularized by Matthew Arnold’s poem, The Forsaken Merman. Various ramifications of this motif include the prohibition against the human wife's overstaying he' visit home, or even seeing her old home or staying in church for the benediction which inevitably prevents her returning to her merman husband. See Havmand; Nakk; Nakki; nix.