Russell Sherwood, 69, of Cilfrew, Neath, went missing while heading to Ewenny, Vale of Glamorgan, after Storm Angus hit in November.
Three days later, his body was found in the River Ogmore.
The Independent Police Complaints Commission (IPCC) is reviewing issues, including the force's call-handling.
Mr Sherwood was reported missing on 20 November after failing to collect his wife from work.
He was found dead on 23 November inside his Ford Focus, which had become submerged in the river.
The IPCC said it would investigate the police response to an earlier report of flooding across New Inn Road in Ewenny on the day of Mr Sherwood's disappearance.
It will also consider "whether call-handlers properly risk-assessed the information provided to them", but not Mr Sherwood's death specifically.
IPCC Commissioner for Wales Jan Williams said: "My thoughts are with the family of Mr Sherwood and all those affected by his tragic death.
"Our investigation will examine police actions prior to his disappearance and whether appropriate policies and procedures were followed.
"We have made contact with Mr Sherwood's family to explain our role and we will be keeping them updated as the investigation progresses."
An investigation has been launched into South Wales Police's response to reported flooding before the death of a missing man.