Have, possibly, too much time on my hands; but, a bit more "mermaidy" material, led to by stuff on another message board, has prompted this post. If it's thought to belong more properly under "folklore" or the like, no doubt mods will act. Incidentally, there's another "mermaids" thread here on "Cryptozoology - general" -- p.4 thereof. It's mentioned there, that the people of Zimbabwe (a slightly unlikely locale for the phenomenon, one feels) are "big" on mermaids.
A kind of aquatic humanoid -- in the US National Ocean Service's phrase -- which I feel that I'd like there to exist (at a safe distance away): the legendary Blue Men, allegedly dwelling in the waters of the Minch, the area of sea between the Scottish mainland and the Outer Hebrides. The following concerning them, is mostly quoted from the book "Sea Room" by Adam Nicolson, about that part of the world.
These beings -- the Blue Men or "Fear Gorm" in Gaelic -- are mer-people said to dwell underwater in the Minch. They are described as strange, dripping, semi-human creatures -- blue-green in colour -- who sometimes, in groups, come to the surface and board boats plying on the Minch. They sit alongside the boat's crew in the stern-sheets, and sing a verse or two of a complex song. Unless the mariners are able to continue the song, extempore, in the same metre and with the same rhyme; the Blue Men will sink the boat and drown its occupants.
There is a tale of a boat crossing the Minch, which came upon a blue-coloured man asleep on the surface of the water. The boat's crew captured him, took him on board, and bound him hand and foot. They were alarmed then, to see two other Blue Men swimming after the boat. These Blue Men spoke to each other thus: "Duncan will be one man", and "Farquhar will be two". This was clearly a threat to the crew; but luckily, before disaster could strike, the captive "broke his ropes, and over the side he went".
It has been suggested that the legend of the Blue Men might be a faint echo, lasting into modern times, of belief in Manannan the old Celtic sea-god; and before him, Poseidon.