It’s pretty well established that faery kind can’t cross flowing water. This is fortunate, as it enables escape by those fleeing them after stealing their possessions, or by those running away from a being that wants to devour them. For example, individuals have got away by this simple strategy from a pursuing boggart, the monstrous nuckelavee on Orkney and a blood thirsty each uisge (water horse) at Trotternish on Skye. In this case some young women let an old female spend the night sheltering with them in their shieling. One awoke in the middle of the night to discover their old visitor sinking her teeth into the arm of another of the sleepers. The first girl leapt up and fled, but the old woman assumed her horse form and made chase. The galloping mare was catching up but- little way from Bracadale church- the girl jumped over a stream and, at the same time, the cocks crowed. The each uisge could not cross the stream and the fugitive was safe. It’s of course extremely puzzling that a water horse has difficulties with water; the explanation would appear to be that the natural habitat of the each uisge is either the sea or freshwater lakes. A flowing river is different, somehow, and because of this forms an impenetrable barrier. The same looks to be the case with the nuckelavee: they live in the sea and can come up on land to ravage the livestock, but are in trouble if they encounter fresh water- which includes rain, oddly enough. One wet days, they won;t come ashore at all…
Then again, a kelpie in human form wanted to cross the River Dee at Inchbare one stormy night. The boatman agreed to take him across- and charged no fee for this. As his passenger departed, a song was heard:
“The Dee shall be quiet and merciful ever
While you and your sons have a boat on the river.”
Perhaps being in the boat was what made the difference for the kelpie, but more likely it was the fact that the kelpie’s native habitat is deep pools in rivers. All the same, why it couldn’t just swim in that case isn’t clear; perhaps it was just lazy, or tired (and why not?).
The Scottish bauchan or bogan (a type of bogie) can cross the sea. In one story from Lochaber a farmer had a love-hate relationship with the bauchan who lived in the vicinity. The pair often used to fight each other, but at the same time the bauchan would gather fuel for the farm in bitter weather and helped the family move house. When the farmer had to leave his land because of the Highland clearances, the bauchan travelled with him to the United States and (in the shape of a goat) helped clear the new land he settled. Perhaps, again, being insulated from the sea by the ship they sailed on was the key thing.
The same ability to cross oceans applies to the Scottish faery-lover, the leannan sith. Evans-Wentz (Fairy Faith 112) recounted how a man from Barra, called Lachlann, had a fairy lover who used to visit him nightly, to the point that he was becoming exhausted by her demands and was beginning to fear her affection. He decided to flee to Canada to escape her, but she quickly found out, and could be heard lamenting by women milking the cattle at evening on the meadows. Nonetheless, when Lachlann reached Nova Scotia, he found the fairy had followed him there. She might have sailed secretly with him, or possibly she might have just used her magical powers to transport herself there. Of course, if she could do that, you begin to wonder why some of the others didn’t. It appears that the inherent defensive properties of fresh flowing water are too formidable even for the faeries’ powers.
For more details of some of the ‘faery beasts’ described here, see my Beyond Faery.
British Fairies 
There’s some connection between faery and Arlo Guthrie’s song (not actually his song as it’s much older, Gypsy Davy. There are older lyrics, using the word ‘glamour’, and it’s typical that a woman was enchanted by a group of musician gypsies and taken away from her home.
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I think that in British folk tradition, both the gypsies and the faeries share this power of glamour. I’m familiar with the song you mention, which I think is a British folk staple: I know it as the ‘Seven Gypsies’ from the album ‘From Here’ by East London folk band Stick in the Wheel.
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