Ho folks- nobody has mentioned the versions fro Maine & New Brunswick so here they are. Also a couple of notes from Fanny Eckstorm regarding her source fro Kerry who mentions the "water witch". She and Flanders collborated occasionally and that may be where Flanders got the story

Lake of Cool Finn / Willie Leonard / Willie Lennox
New England versions
From British Ballads from Maine Volume 2, Phillips Barry, Fanny Hardy Eckstorm, Mary Winslow Smyth, edited by Pauleena MacDougall, Published by Northeast Folklore, Volume XLIV, Maine Folklife Center, Univ of ME, Orono, ME, 2011

WILLIE LEONARD, or, THE LAKE OF COLD FINN Eckstorm, BBM2, Mr. Thomas E. Nelson, Union Mills, New Brunswick January 1929
WILLIE LEONARD, or, THE LAKE OF COLD FINN Eckstorm, BBM2, Fred W. Morse, Islesford, Maine, January 22, 1934 (also noted by Flanders in Springfield, Mass. Sunday Union, September 9, 1934)
WILLIE LEONARD, or, THE LAKE OF COLD FINN Eckstorm, BBM2,Mr. Horace W. Priest, Sangerville, Maine August, 1925 as learned by him in a lumber camp in the 1880s.
WILLIE LEONARD, or, THE LAKE OF COLD FINN Eckstorm, BBM2 Mr. Howard S. Gotchell, St. Stephen, New Brunswick.
Mr. Fred Nesbitt, of St. Stephen, New Brunswick,
WILLIE LEONARD, or, THE LAKE OF COOL FINN,Phillips Barry, Maine Woods Songster 1939 Phillips Barry on Willie Leonard, or The Lake of Cold Finn (Bulletin of the Folk Song Society of the Northeast 8. Cambridge, Mass.: Powell Printing Co., 1934) o10, II.

Lake of Cool Finn/ Flanders (as noted by Eckstorm) Mrs. Herbert Haley, Cuttingsville , VT
Lake of Charlin /Flanders 2069 Mrs James Cook, Bennington VT c.1940
Lake of Cool Finn/ Flanders 2070 Belle Luther Richards, Colebrook NH c.1940
Lakes of Col Finn / Flanders 2075 Mrs Ellen Sullivan, Springfield VT c.1940 same melody same as Mr. Fred Nesbitt, of St. Stephen, New Brunswick, trans by his son D.A. Nesbitt BBM2
----------------
According to Eckstorm BBM2 p 69
"A correspondent who was born in County Kerry writes Mrs. Eckstorm that she heard "Young Willie of the Lake of Cool Finn" sung in her childhood. "The tale of the legend (the old people said it was a true tale) is, a mermaid fell in love with young Willie. There is one time in the year when the mermaids have power over mortals. She had it that day when she waked young Willie up and had him come to the lake. The first time he swam round, he was too strong for her and he swam to an island...His comrade didn't want him to go in again:
Young Willie rested and said "I'll go-"
The waves they were rough and the wind it did blow;
"Willie, dear Willie, don't you go in,
There is deep and false water in the Lake of Cool Finn."
The second time he swam around, she pulled him in. His body was never found, and it was a dummy body they had in the casket. So the tale goes." We do not attempt to identify the actual lake: the mention of Lough Sheeling (Maine C, Shorlin, Grieg, Folk-Song of the Northeast, CXIV, Shilin) may be due only to the singer's perplexity, as is the mention of Lake Champlain in Vermont A."
None of the New England versions contain this "mermaid" detail.

p. 75
"Mr. Samuel P. Bayard, Pittsburgh, Pennsylvania, who has done extensive field work in southwestern Pennsylvania and northeastern West Virginia, has found "Willie Leonard" in several forms well established in popular tradition in this locality. It has not to our knowledge hitherto been reported from any part of the Southern Highland."

VT version- Lakes of Champlain
http://www.itma.ie/goilin/song/lakes_of_coolfin_sara_grey