Authorities said a bystander spotted a group of people in a boat near Rocky Point Road at about 8 a.m. Sheriff's deputies and immigration officials responded to the scene and found a 25- to 30-foot open hull boat.
Fifteen men and two women were detained at Lunada Bay, believed to be part of a human smuggling operation.
"We will try to determine who the actual alien smugglers were on the vessel and then those individuals will be presented to the U.S. Attorney's Office for federal charges," said ICE Agent Jeremy Scott.
There is no evidence of drugs, authorities said. Four people with minor injuries were taken to a local hospital for treatment. Officials say some were hypothermic.