Here it is, from "Ballad Book" edited by Katherine Lee Bates, published 1890, found at Project Gutenberg. I think the Scots is a bit more authentic, and it has a couple more verses.

LADY ISOBEL AND THE ELF-KNIGHT.

There cam' a bird out o' a bush,
    On water for to dine,
An' sighing sair, says the king's daughter,
    "O wae's this heart o' mine!"

He's taen a harp into his hand,
    He's harped them all asleep,
Except it was the king's daughter,
    Who ae wink couldna get.

He's luppen on his berry-brown steed,
    Taen 'er on behind himsell,
Then baith rede down to that water
    That they ca' Wearie's Well.

"Wide in, wide in, my lady fair,
    Nae harm shall thee befall;
Aft times hae I water'd my steed
    Wi' the water o' Wearie's Well."

The first step that she stepped in,
    She stepped to the knee;
And sighing sair, says this lady fair,
    "This water's nae for me."

"Wide in, wide in, my lady fair,
    Nae harm shall thee befall;
Aft times hae I water'd my steed
    Wi' the water o' Wearie's Well."

The neist step that she stepped in,
    She stepped to the middle;
"O," sighend says this lady fair,
    "I've wat my gowden girdle."

"Wide in, wide in, my lady fair,
    Nae harm shall thee befall;
Aft times hae I water'd my steed
    Wi' the water o' Wearie's Well."

The neist step that she stepped in,
    She stepped to the chin;
"O," sighend says this lady fair,
    "I'll wade nae farer in."

"Seven king's-daughters I've drownd here,
    In the water o' Wearie's Well,
And I'll mak' you the eight o' them,
    And ring the common bell."

"Sin' I am standing here," she says,
    "This dowie death to die,
Ae kiss o' your comely mouth
    I'm sure wad comfort me."

He's louted him o'er his saddle bow,
    To kiss her cheek and chin;
She's taen him in her arms twa,
    An' thrown him headlong in.

"Sin' seven king's-daughters ye've drownd here,
    In the water o' Wearie's Well,
I'll mak' you bridegroom to them a',
    An' ring the bell mysell."