LA FAYETTE, ALABAMA -- James Michael Grimes is speaking out for the first time in an exclusive interview with ABC News after going overboard on a Carnival cruise ship after being missing for almost 20 hours.

The 28-year-old man said he was determined to make it out of the Gulf of Mexico alive, calling it the experience of a lifetime.

Grimes said he treaded water for nearly 20 hours after falling overboard on Thanksgiving Eve -- battling jelly fish, rip currents and shark-infested waters before being airlifted by the U.S. Coast Guard on Nov. 24. shortly after 8 p.m., the U.S. Coast Guard told ABC News.

Grimes had only been aboard the Carnival Valor for a day before his sister reported him missing. The two were last seen together at a restaurant where Grimes had won an air guitar contest before going to the bathroom.

That is the last thing he remembers. Grimes said he believes the fall overboard knocked him unconscious.

"The next thing I know... I regained consciousness. I was in the water with no boat in sight," he said.

Alone in solid darkness, and the light from the stars and the moon, Grimes decided in that moment he would make it out of the water.

"I felt like I was given a chance right then... you're alive for a reason... that [fall] could've killed me, but I felt like from that moment on, I was trying to stay positive. And, you know when you're here, you're still alive for a reason. So, all you got to do now is swim and survive. I was hoping... they will start looking for me... they will find me eventually," Grimes said.

Grimes was aboard the five-day cruise with 18 of his family members for Thanksgiving. When he didn't return to his cabin that night, his sister reported him missing.

The ship was on its way to Cozumel, a Mexican island in the Caribbean, and was released by the Coast Guard to continue to its destination after the rescue was made.

"The Jayhawk aircrew hoisted the man onto the helicopter and transferred him to awaiting emergency medical services at the New Orleans Lakefront Airport," a statement from the Coast Guard read.

"We are beyond grateful that this case ended with a positive outcome," said Lt. Seth Gross, a Coast Guard Sector New Orleans search-and-rescue mission coordinator.

"We greatly appreciate the efforts of all, most especially the U.S. Coast Guard and the mariner who spotted the guest in the water," the Coast Guard said in a statement Thursday to ABC News. "Cruise ships have safety barriers in all public areas that are regulated by U.S. Coast Guard standards that prevent a guest from falling off. Guests should never ever climb up on the rails. The only way to go overboard is to purposefully climb up and over the safety barriers."

Carnival Valor had said in a statement to ABC News that it conducted a search-and-rescue operation after Grimes went missing.

"Carnival Valor retraced its route to support the search and rescue, but the ship has now been released by the U.S. Coast Guard, and will continue on its way to Cozumel. Carnival's Care Team is providing support to the family members of the missing guest who were sailing with him and remain on board," the statement at the time said.

Against all odds, Grimes said he kept a positive attitude and "just kept swimming."

In addition to his attitude, he said it was his will to see his family again that kept him afloat.

"I wanted to see my family and I was dead set on making it out of there. I was never accepting that this is it, this is going to be the end of my life. I'm 28 years old. I'm too young. This is not going to be it," Grimes said.

"I always thought there's a greater purpose for my life. Now, I know for sure I'm meant to do something on this Earth. And, you know, I don't know. It was just the Lord was out there helping me, giving me strength and helping me stay afloat," he added.

ABC News' Victoria J. Arancio contributed to this report.