Craig Windebank, 14, rescued Sonny Wells after the 20-year-old jumped 30ft from South Parade Pier in Southsea into just three feet of water.
Mr Wells, from Waterlooville, has been told he may never walk again after damaging his spinal cord.
Craig and his pal Si Massiah, 15, were sunbathing on the pier at the time of the horrific incident on Sunday.
Craig ran into the water when he saw Mr Wells floating face down in the sea and being carried by the current under the pier.
Craig, of Queens Road, North End, Portsmouth, said: 'I was lying on my front sunbathing when I heard my mate shout, "he's drowning".
'I didn't even think. I just thought, "he's drowning and I've got to help him".
'I couldn't believe it because there were lots of people on the beach but they just stood and watched.'
Craig swam out to Mr Wells. With the help of his friend Si and another man, he carefully brought Mr Wells back to shore.
Craig, who attends St Edmund's RC School in Portsmouth, said: 'His head kept going under water so I had to hold it really carefully.
'I thought he might have broken his neck because his neck felt really floppy.
'He wasn't breathing, but we found a pulse. I think another couple of minutes and he would have drowned.'
Craig then waited with Mr Wells on the beach until the severely injured man was airlifted to the specialist neurological ward at Southampton General Hospital by helicopter.
Craig, who said he had been having nightmares about the incident, added: 'I don't feel like a hero, although that is what people have been saying.
'I just did what I had to do. The main thing is that this guy is still alive.
'I don't even know him, but I really hope he's going to be okay. I would like to go and see him in hospital.'
Mr Wells's family are at his bedside and have been told he will remain in hospital for the foreseeable future.