I hadn't realized until recently that mythological stories went under "Books" here, and I certainly wasn't expecting to see a story featuring Jenny Greenteeth, who I always thought was one the creepier folktales out there.
As always, you're very good at quickly establishing characters and making me feel like I know them, and here this includes both Zach and, to an extent, Jenny herself. The word count limit kind of hurts how much can actually happen, as well as how much the water hag's slow, creeping menace can be emphasized, but within those confines I think you did quite well.
As always, there's a bit of humor in here as well, with Zach's hilarious misunderstanding of what's going on. Sure, there's a horrifying flesh-eating thing crawling out of the pond toward him, but she must just be angry that he moved that rock!
Finally, I really liked the little detail of the weed covering the pond. Not only does it serve to hide Jenny, but it incorporates the fact that apparently Jenny's name is often used as a word for duckweed and over water-covering plants.
The description of why and how he got to be there was good, and I could pretty much feel the heat and exhaustion as he trekked with all of his gear in the August heat. Then the description of Jenny herself was pretty neat, especially that she'd have a name in the first place. (I don't think I've ever heard any J-Lo songs unless I happened to see a commercial.) Some of the references lost me ("high on the three balls smashed past Newcastle"), but I guess I'd have to know more about Britain and football to get it all.
Still, it was a fun little tale of how he got to be in the wrong place at the wrong time. I guess Jenny was lucky too that he didn't accidentally toss the big rock onto her.