He rushed to the water where he saw two men struggling to stay afloat.

"I was ready to jump but, look, I didn't jump because there was already someone in the water with him, so we just decided we'd just pull him out," he told Ten News.

Police believe a man in his 20s jumped into the water off Harbour Esplanade to rescue another man in his 50s about 9pm on Sunday.

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The younger man also began to struggle and Mr Grimes helped pull both men out.

"I grabbed him under the arm. I was afraid I was going to fall over so I got one of the girls to actually stand on my feet so I didn't fall over. It took a while to lift him up out of the water and then lay him down on the pier," he said.

The man in his 50s was semi-conscious and Mr Grimes put his hand on the man's stomach before moving it up to his chest.

"He just started coughing and we turned him over to reposition," he said.

"The real hero was that guy who jumped in. He took the basic instinct of running and jumping in."

The man in his late 50s was taken to hospital in a serious but stable condition.

The younger man was also taken to hospital with non-life threatening injuries.

A Victoria Police spokeswoman said the incident was still being investigated.