With help from her friend Sam Grapes and another man, Chris Sparkes, the rescuers managed to haul the man from the water and into the back of the boat.
Taking control of the situation, Ms Warren performed a series of compressions on the man, managing to force out of his lungs the water he'd swallowed.
The rescuers then shifted the man into the recovery position, from where he was able to force himself to breathe.
A crowd of onlookers climbed on to the boat, some taking pictures with their phones, but were soon ordered away.
"With the number of people standing around screaming, it was hard to get through to the ambulance people and tell them he was actually in quite a serious condition," Mr Sparkes said.
"He was making noises and grunts, there was foam coming out of his mouth, and he had quite a big gash on his head ... there were actually a few moments when we thought he'd gone."
When emergency services arrived, the man was quickly loaded on to a stretcher and rushed to hospital in a critical condition.
He remains in a serious condition.
Some time later, Ms Warren received a message that the man had regained consciousness.
Napier police senior sergeant Dan Foley, a former lifeguard himself, said Ms Warren's presence would have made a big difference.
"We are really pleased there were people there who had expertise in first aid. It would appear the outcome was much better as a result of it."
