I definitely like Butsaay's interpretation the best, as it makes for the most poignant story, but what gives me pause in believing that is the title of the animation.
"The Visitor" wouldn't make a very smart title if the monster were in the boy's imagination... unless "The Visitor" was the boy himself, visiting the lake, but that would strike me as a tad melodramatic. Titles are emphatic, so a visitor in this case would denote significant travel, not just down the street where you can run to the place from your house in a fit of teen angst.
Or I could be totally off and, like you said, "The Visitor" is the boy's imaginary friend... just not coming from as far away as that title would suggest. On another note, the animation style for the monster reminded me of "Blue" from Foster's Home for Imaginary Friends.
Five stars, because hashing out the source material is really fun and I almost never see an NG original prompting a thoughtful discussion like this one.