The former Cleveland Browns running back detailed the Jan. 4 rescue mission in an interview with "Good Morning America" ... where he explained there were no warning flags near their spot at Pensacola Beach prior to the incident.

Hillis said he immediately sprang into action and ran into the water as soon as he found out the kids were in danger -- and he had an impossible decision to make.

"I think the scariest point ... was when I'm swimming to my son and I have to pass by him because my niece is in more danger," Hillis told Michael Strahan. "I knew that I had to pass him up to get to Camille first. Because, you know, if I didn't then there's no way she would've made it."

After getting his niece to safety, Hillis said by the time he was back in the water, his son, Orry, was out of energy and couldn't swim anymore ... and he was struggling to stay afloat himself.

In fact, Hillis said he saw his son's eyes roll in the back of his head ... and he began to pray as he fought his way back to shore.

"Lord, please," Hillis said. "Like, I really don't care about my life. I don't. But if we can get him out of here that’s the only thing I’m going to care about."

Thankfully, Hillis was able to get Orry to safety ... but that's when the former Madden cover athlete started to lose consciousness -- and his kidneys and lungs began to fail.

Hillis was airlifted to a nearby hospital, where he spent more than a week in the ICU ... and he said the road to recovery is long.

"I don't think my lungs will ever get back to where they needed to be," Hillis continued. "Physically, it's coming along okay. Mentally things [are] coming along a lot slower. I'm just trying to take it one day at a time."