Other people have told him someone then jumped off the wharf and landed on top of him.
"I can't remember much. What I do remember is looking down at myself drowning for about 20 seconds, then waking up on the wharf for about 10 seconds then blacking out again, and then being in and out of consciousness all the way to the hospital.
"Waking up in the hospital was pretty scary because I didn't know why I was there. All I knew was that I was in the hospital and I was hurt."
Josiah's top teeth had gone right through his lip, requiring three stitches, and he had scratches all over his back from people pulling him back to the safety of the wharf.
He was kept in Tauranga Hospital overnight on Sunday but discharged himself about midday on Monday.
The out-of-body experience was particularly frightening, he said.
"It's an indescribable feeling. I was looking down on myself slowly dying. I could feel myself choking but I couldn't do anything. It felt like my mind was still there but my body wasn't.
"I'm just glad that someone did save me, even though I don't know who it was. I'm just glad that they reacted quickly enough to save me."
Neale Mann was at the nearby Mount Ocean Sports Club when the drama unfolded. He believed someone had jumped into the water and landed on Josiah's head.
A couple of Josiah's friends held his head above the water until two adults jumped in and helped drag him out, he said.
Mr Mann, an ex-ambulance volunteer, called 111 and put Josiah into the recovery position until paramedics arrived.
"He was semi-conscious. He didn't know what the hell had happened. It could have been worse. He could have quite easily drowned."