A witness who saw the boat capsize about 100 metres from the shore at 10am went to the rescue of the older man, who had started swimming towards the beach.

Acting Sergeant Danny Lean of Mornington police said the older man from Mooroolbark was not wearing a lifesaving jacket. He said CPR was applied to the man on the beach but was unsuccessful.

Acting Sergeant Lean swam out to rescue the second man, who had let go of the upturned vessel.

He said the Wantirna man was unable to swim and was screaming for help. "I swam out, grabbed him and brought him back in. The air wing and water police were going to be five to 10 minutes."

Acting Sergeant Lean said the man was wearing a safety jacket [but] "I couldn't just stand there and watch him".

He said that before the boat tipped over, a witness had seen one of the men move to the back of the vessel because the rear was "riding low" and the witness thought it appeared as if the shift in weight caused it to capsize.

Paramedics treated the Wantirna man for hypothermia and he was taken to Frankston Hospital for observation. Water police are investigating the incident.

Sergeant Lachlan Holding of the water police said the men launched their boat at Olivers Hill boat ramp at 5am to go fishing.

He said witnesses told police the men had brought their boat to shore near Freemans Road, Mt Eliza, before it capsized, because they were having difficulties with the vessel.

They then took it back on to the bay in an effort to get it back to the ramp at Olivers Hill when they again struck difficulty a short distance from the Freemans Road beach. "Danny did an excellent job in trying circumstances," Sergeant Holding said.

In a second incident on the same day, an 81-year-old woman died while swimming at McCrae beach.

Ambulance paramedic Travis Jones said the woman had been seen floating face down in the water at 2pm.

"Bystanders pulled her to the sand. She was unresponsive so they attempted CPR," he said.

Police are investigating and will prepare a report for the coroner.