Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Amber Heard stars as Mandy Lane, the hot girl all the boys want, in this teen slasher movie about a group of good-looking teenagers who go to an isolated Texas ranch house to party for the weekend, but a mysterious man kills all of them except for the final girl. The entire plot is a horror movie cliche, and this one is loaded with them, including the fact that there is a twist at the end, but unlike most, this twist involves the final girl, who turns out to be a psycho killer herself.
After getting to the ranch, everybody jumps in a lake for a swim. Mandy shows up after going jogging so she's the last one in the water. Right after she dives in we see underwater shots of a snake crawling into the water and swimming across the surface. We see a snake's-eye view of Mandy in the water, then the snake, then Mandy again as the snake gets closer to her. The snake appears to be going after Mandy (just like the equally cold-blooded males in the water.)
The ranch hand Garth suddenly appears at the edge of the pond with a rifle and shoots the snake just in time before anyone gets hurt. Nobody in the pond sees the snake until they hear the gunshot and find the dead snake in front of them. Garth tells everyone that they should always throw a rock in the water before going swimming, to give the snakes fair warning. Everybody thanks him as they get out of the water, in too much shock to continue swimming.
The scene establishes a sense of danger on the ranch, that there are things out there that could kill them, and it introduces us to Garth and his rifle. When the murderer starts killing them we are meant at first to suspect that Garth is the killer. Later we see the real killer shoot someone in the water with a rifle at the same swimming spot.
The snake we see swimming is a live bull snake, a common harmless snake found in Texas, so they got that right. Then we see a fake snake floating in the water that is supposed to be the dead snake. There should also be blood in the water, but they didn't bother to show any. Garth must be a great shot to hit a thin moving snake without blowing someone's head off. Shooting a gun into a crowd of moving people is probably far more dangerous than just leaving the snake alone. A snake won't usually swim up to someone who is not attacking it to bite the person unless the person is a potential meal. But that doesn't mean it won't get scared and bite them anyway so we can't blame Garth.