Tha deagh-fhortun agaibh. By happy coincidence, I happen to have borrowed a book from the local library which has info. on the each-uisge:

Ethel Bassin. 2The Old Songs of Skye: Frances Tolmie and her Circle". London: Routledge and Kegan Paul, 1977.

p.35-36

"From Kate MacDiarmid (Catriona Mhòr, who had led the singing with much gusto at the waulkings, Fanny learned and noted 'Cumha an each-uisge' (Example 9).

"The each-uisge (water-horse) is a familiar figure in Gaelic folklore. In the guise of a man he might lay his head in the lap of a girl and invite her to 'dress' his hair. The tales vary between the girl who does, or does not, discover in time from the sand in his hair and on his breast what he really is. Mòrag in this instance, does not make the discovery until after she has borne him a child. In terror she flees, leaving the baby with him. This song - a lullaby still current in oral tradition - is the each-uisge's entreaty to Mòrag to return, alternating with his affectionate lulling of the child.

"In the Journal Miss Tolmie remarks that her pleasure in these old wive's songs was considered very odd by her contemporaries, 'for they were not deemed "poetry" or worthy of notice by song-collectors of that period'. Some of her elders, fortunately, were of her own way of thinking, notably her aunt, Mrs Hector Mackenzie (Annabella Tolmie), whose only son, John Tolmie Mackenzie was harbour- master at Dunvegan as well as being factor to MacLeod of MacLeod."

example 9 in Bassin's book, from Tolmies Journal, 7, collected from Kate Macdiarmid, cottar, Minginish, Isle of Skye, 1862

staff notation is in the book - slow, c# and F#- D/AAAF/DBAF/EEEE/EEE//refrain: DFE/DFED/AAF/f-AAD/d-bAFE/EFE/EFE [this doesn't show you timing, but you should be able to tell if it's similar to the air you play]

A Mhórag dhonn! A Mhórag dhonn! Till gud' mhacan; 'S gheibh thu 'm bradan breac o'n loch.

chor: A-hó hi. A-hó hi. A-hó hó-an, A-hó hó-an, A-hó hi. A-hó hi.

Tha 'n oidhch' an nochd
Gu fliuch frasach,
Aig mo mhac-sa ri sgàth cnocain.

Gun teine, gun tuar,
Gun fhasgadh
'S tu sìor chonràn.

Mo shean-a chab liath.
Ri d' bheul beag baoth.
'S mi seinn phort duit am Beinn Frochdai.

O brown haired Morag, come back to thy little son, and thou shalt get a speckled salmon from the loch.

The night is wet and showery for my son in the shelter of the knoll without fire, pale, forlorn, and wailing without cease.

My unsightly old grey mouth, against thy silly little mouth, while I sing dandling songs to thee in Ben Frochdai.
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- I give the translation as in the book. If you plan to devise a singable translation, there's no particular reason to use 'thou' and 'thy'
- a footnote says "This song appears as 'Oran an t-Each-Eisge - Nuair Theich a Bhean Bhuaidh', 'The water Kelpie's song when his wife left him', in 'Gesto', p.20 [MacDonald, The Gesto Collection]
- In 1897, Tolmie collected 'Oran-tàlaidh an eich-uisge' (lullaby of the water-horse)[ this title uses the standard genitive - 'eich'/ of the 'each' - horse] and 'Caoidh an eich -uisge' (Lament of the Water-horse) both from Mary Ross, Killmaluag, Skye (so apparently two distinct songs)

I've recently heard Skye residents talk casually of a water-horse in a loch in the Sleat peninsula, and one enterprising soul has erected a large skeleton purported to be of an "Equs aquea" (or something like that|)

The mermaid/seal-human stories are a bit different from kelpie stoires. It's nice that the tunes about the water horse and the selchie go well together! and in Bassin's book the next song given, collected by Tolmie from her aunt Mrs Mackenzie, is a mermaid song 'oran mu'n Ghruagaich-mhara'. The singer describes seeing the sea-maiden in a grey robe, stretching and changing her appearance to that of 'an animal without horns' - patently a seal. There are several different songs on this theme - the Irish one Annraoi mentions (recorded by Clannad and Altan) has a child singing of the mother who returns to the sea, while in Oran an Mhaighdean-Mhara (recorded by Ishbel MacAskill/NicAsgaill), the sea-maiden herself sings. The 'great silkie' or 'selchie' song as I've heard it is unusual in that the seal is a man, a father, rather than a mermaid.

Bassin quotes W T Dennison, Orcadian Sketch-book : "every true descendent of the Norseman looks upon the seal as a kind of second-cousin in disgrace."

It might be worth looking up the School of Scottish Studies, Edinburgh website to see if they have any kelpie and/or selchie tales. http://www.pearl.arts.ed.ac.uk/