Four divers were scouring St. Anns Bay in Cape Breton for the sunken car, which plunged into the water the night before, RCMP Sgt. Al LeBlanc said.
"If it was a lake and there was no current, it would obviously be a totally different environment," LeBlanc said in an interview from Halifax.
"The last thing we want to do is put our officers at risk. They're looking for the vehicle, but at the same time we have to make sure it's safe for them to do so."
Crews from several fire departments were also searching for the car along the shoreline.
LeBlanc said it was unclear how many people were in the car when it entered the water because it was dark at the time and workers aboard the provincially run ferry could not see inside.
He was unaware of any reports of missing people from the area.
Police received a call Monday at around 8:45 p.m. about a car that had driven onto the cable ferry on the north side of the bay, sped up and then drove off the end of an upright ramp.
The car went into the water about 20 metres from shore and drifted for about 200 metres before it sank, LeBlanc said.
"The vehicle drifted across for some time and then basically went into the water."
He said it was too early to say what caused the car to drive off the ramp because the incident happened so quickly.
"The reality is that we really don't know," he said.
Pam Menchenton, a spokeswoman for the provincial Transportation Department, said there is no manifest listing passenger names or a surveillance camera on board, though crew members check for passes or tickets once vehicles come to a stop.
The cable ferry, which links the small communities of Englishtown and Jersey Cove, began running in 2008 and is among seven provincially operated ferries. The crossing only takes a few minutes and the ferry operates 24 hours a day, depending on demand.
The provincial government said it was co-operating with the RCMP and had taken the ferry out of service until further notice.
The government said it would also interview the ferry's crew members and inspect the vessel as part of its own internal investigation.
In a news release, the Transportation Department described the ferry as one of the newest and busiest ferries in its fleet.
Menchenton said drivers would have to take a 15- to 25-minute detour through Highway 105 or the Cabot Trail while the ferry is out of commission.
Note to readers: This is a corrected story. An earlier version misspelled LeBlanc.