The collision happened on Saturday night near Arizona's border with California when a boat with 10 people and a craft carrying six people collided. Several people were rescued by nearby boaters, the Mohave County Sheriff's Office said.
"This is technically a diving mission looking for potential drowning victims," Sheriff Doug Schuster said at a news conference. He said it was possible that the missing - one male and three females - had made it to the shore.
The cause of the accident was under investigation, Schuster said.