Investigators say the accident happened around 3:40 Thursday morning when an initial hit-and-run occurred.
The 18-year-old driver of a car told police he was heading eastbound when his car was hit by a semi, which then drove off.
A Harris County deputy was taking the report when the driver of a second 18-wheeler swerved to miss the crash scene.
That driver crashed through the guardrail and plunged 20-to-30 feet into the water.
Sheriffs are searching for the driver but they say there are no signs the driver escaped the crash.