Rebecca Weiss, 50, headed out in the morning but failed to return home by the agreed time of 5pm, according to her husband Alan Weiss.
Her brother says it's hard to believe something could have gone wrong.
'My sister is a strong swimmer, and she was wearing a wetsuit, according to my brother-in-law,' brother Bob Causey said.
'So it would have to be something out of the extraordinary for something to happen to her out there.'
Homicide detectives have interviewed Alan Weiss but say he is not considered a suspect.
The case was turned from a missing person investigation to a homicide case.
'There has to be somebody who must have seen,' said mother Vilma Causey.
'This is not an isolated place. This is a resort. There should have been people.'
Her clothes and other belongings were found near her BMW X5, which was parked at the Terennea resort on the Palos Verdes Peninsula, near Los Angeles, California.
Mr Weiss, who co-owns two grocery businesses with his wife, who is described as 5ft 5in tall and Asian, says he spent four hours looking for her.
He reported her missing at 11pm on Saturday and today she still remains officially missing. Read More