Thurston County Undersheriff Brad Watkins said no bodies were found in the car, which was discovered Sunday beneath the surface of North Gull Harbor, in an area called Little Fishtrap.
The missing woman was identified late Sunday as Shantina M. Smiley. Her son was identified as Azriel Carver.
Thurston County Sheriff Lt. Chris Mealy said the woman and her son were traveling from Silverdale to see her stepfather in Castle Rock, but they never arrived at their destination.
The Sheriff's Office is treating the case as suspicious, but detectives have very little information to go on, Watkins said.
He said there are no obvious signs of foul play, but they want to know how where Shantina is - and how her car wound up in the water.
Investigators have determined that Shantina left Silverdale at 5 p.m. Saturday. At 6:45 p.m. she called her boyfriend from a pay phone asking him to get her cell phone, but he didn't answer, Mealy said.
Detectives found a receipt at a local business that places her in northeast Olympia at 7:55 p.m.
At 11:15 a.m. Shantina's car was found submerged in North Gull Harbor.
Her wallet was found inside the car with everything in it, but not her purse, Mealy said. The doors of the car were open when it was discovered.
Mealy said Shantina has no history of mental illness or any other known problem. She works as a portrait photographer in the Silverdale area.
Contrary to earlier reports, she was not visiting a friend in the area, she was just heading to her stepfather's home in Castle Rock.
Investigators conducted a boat search of North Gull Harbor and the immediate area, as well as the beach area, but found no trace of Shantina or her son. Deputies also are canvassing the neighborhood.
The investigation continues.