Robert C. Langendoerfer, 22, admitted on Monday that he would likely be found guilty of first-degree manslaughter and witness intimidation in the death of 17-year-old Britney R. Galindez.
He entered an Alford plea, meaning he does not admit guilt but realizes he would likely be convicted.
Prosecutors agreed to the plea deal because they feared witnesses could change their stories when put on the stand if the case went to trial.
The victim's family was not happy about what they say was a short sentence.
"There's nothing that will bring her back or ease the pain that you've caused her whole family," said Britney's stepmother, Brandee Bittick. "It's insulting to know that one day you'll be free to live your meaningless life."
Galindez was found dead in Lake Ballinger in October 2008, days after her family reported her missing.
According to investigators, Galindez was with Langendoerfer and three other people, drinking and riding in a car when she vomited, upsetting others who were in the vehicle.
The driver stopped at the Lake Ballinger boat ramp and everyone walked toward the water, court documents said, where Langendoerfer got on top of Galindez, stabbed her in the neck with a screwdriver and threw her into the lake.
The police report said witnesses told detectives that Langendoerfer then waded into the lake and held Galindez underwater for about five minutes until she died.
Langendoerfer allegedly told those who witnessed the killing that if they told anyone what happened he would kill them, too.