The unnamed man, 30, has severe bites all over his body after being attacked by the pet mastiff cross and a Staffordshire bull terrier in East Sussex.
His injuries were so severe he passed out but the viscous canines continued to attack him east of the city's famous Palace Pier.
Jack Le-Flay, 20, who saw the attack, said: "I was returning home after a night out and saw the attack. We could see these two dogs dragging something out of the water.
"At first we thought it might have been a police training exercise or an old tyre but then we could see a leg.
"The dogs would run back to their owner and then rush back and start attacking the man again. There was blood all around one of their mouths.
"There was a lot of blood on the man who was lying face down in the water. I have never seen a dog behave like that before. It was barbaric."
Police including an armed response team tried to subdue the animals with one officer suffering hand and leg injuries after being bitten.
Paramedics could not get close to the man until the dogs, who are now in police kennels while the incident is investigated, were captured.
A 40-year-old man from Brighton was arrested at the scene on suspicion of being the owner of the out of control dogs, on suspicion of assault, possession of a knife and possession of Class A drugs.
The victim is in a critical but stable condition in the nearby Royal Sussex County Hospital.
Detective Inspector Ian Still, of Brighton and Hove CID, said: "This appears to be a very nasty attack. We are still in the early stages of our investigation.
"We are still trying to work out why this incident has taken place. The dogs were still aggressive when police arrived and officers weren't able to approach the victim initially."