It’s natural that the family are going to want to keep hoping while her body hasn’t been found.
In the absence of a body they will always hope that the police are wrong and that she could still be alive. But that doesn’t mean that she is. It just means that she hasn’t been found.
It can take weeks for a body to be found in water. If ever. If she had been swept out to sea she may. Never be found.
People often feel the same when a loved one e.g. dies in a plane crash and can never be found.
I know someone who died in the plane crash in Indonesia in the early 90’s, where the pilot killed himself and the passengers by deliberately plunging the plane into the ground. None of the bodies were recogniseable, and his wife said she kept thinking maybe he was wandering around looking for help. He wasn’t, but the fact his body could never be found meant she didn’t have that closure.
Hopefully her body will be found soon and then the family can grieve properly.