Ooh, good. I'm very curious to see where you take that. England, particularly in Northumbria and the Fens, has a lot of such folklore, some of it very dark indeed. Have you seen Terry Pratchett's delightful take on that myth in 'The Wee Free Men'? Or the Cloudstreet song, 'Jenny Greenteeth' which follows the Cornish and southern English variant of the water demon as a beautiful green-haired woman who lures young people into the water and drowns them? 'Come into the water and bathe, my love/come swim in the swirling foam/come down in the deep with the rocks and the bones....' Brrrr!
Then of course there are the Cornish merrows. Interestingly, with them, the females are the classic harmless decorative mermaid rather than the dangerous kind, and it's the males, the merrows, who are the predatory green-haired human-eating monsters.
Michael Scott Rohan has an interesting modern take on the related Peg Powler tale - well worth reading. She turns up as a peripheral but thoroughly nasty character in the series of his novels that starts with 'Chase the Morning'.
