Before Sarah and Elijah tackled the boat launch, the strange girl calling herself Branwen ran off and didn't return. This left the two of them "...putting out to sea in a beautiful pea-green boat..." (Sorry, I went back to a rhyme from my childhood--the story of the Owl and the Pussycat.) Actually, they launched the old and far from beautiful wooden craft onto the still dark waters of a mysterious lake.
Out in the middle of the water, with one oar and left up to whatever current there is, Sarah points out the lake seems to end up ahead, but there are no signs that it actually goes anywhere, "like an infinity pool", Sarah points out. It does look as if their boat, with them in it, will simply "fall off the end."
Elijah realizes he will have to get out of the boat and push it toward the bank, since the only oar isn't helping keep them on course. Sarah closes her eyes. She hears the splash as Elijah climbs out of the boat; but when she opens them, he has disappeared...
The apparent end of the lake is coming closer. What happens at the end--does the water end? Does the boat, with Sarah in it, simply drop, as a waterfall over a cliff? Why can't she see anything beyond? And how did the thick, white, fog envelope the boat so quickly--now she can't see anything at all.