Brief partial synopsis from Wikipedia:
Maddy, a marine biology student, returns home for the summer to the water park she co-owns and is annoyed to find that the other owner, her stepfather Chet, plans to add an adult-themed section to the water park that allows nudity. Later that night, Maddy's friend Shelby and her boyfriend Josh go skinny-dipping in the lake, where a piranha fry enters Shelby's vagina. Their friends Ashley and Travis begin to have sex in their van, but Ashley accidentally releases the parking brake, causing the van to roll into the lake. Handcuffed to the van during foreplay and unable to escape, Travis is devoured by piranhas while Ashley, on the roof, calls for help. No one is around to rescue her. The van sinks into the lake and the piranhas eat her.
Maddy consoles Shelby about their friends, and then a school of piranhas attacks them. They manage to kill one, and Maddy, Kyle, and Barry[clarification needed] take it to marine expert Carl Goodman to examine. He informs them that the piranhas may be moving between lakes and could evolve to travel over land. The trio returns to the lake, where they establish that the piranhas cannot enter the pipes connecting the lake and the water park. While Shelby and Josh are having sex, the piranha in Shelby's vagina bites Josh's penis, forcing him to chop it off. Both are hospitalized but survive. Kyle is revealed to be corrupt and taking bribes from Chet, who is pumping water from an underground river into the water park. Chet orders Kyle to keep Maddy from finding out about his plans.