Named Marina after she was discovered at death's door in San Diego's Marine Room restaurant in February, the pup was then rescued and nursed back to health by SeaWorld staff.
"The odds were really against her," Jody Westberg, supervisor of SeaWorld's rescue team, told InsideEdition.com.
At 8 months old, Westberg said Marina weighed just 20 pounds, or almost as light as a newborn sea lion pup.
"She was like a walking skeleton," Westberg said. Nonetheless, the pup was feisty.
And Westberg said she kept that sassy attitude as she entered rehabilitation.
Rescuers first got Marina hydrated. They then fed her what is essentially a fish smoothie to help get nutrients back into her system.
From there, Marina was soon able to start eating fish again. Sooner, in fact, than anyone expected.
In just two months, Marina more than doubled her size.
Then, on Tuesday, Marina's eight months of rehab came to an end as rescuers returned the young sea lion to her Pacific home.
In an area off the coast of Southern California known to have plenty of the fish that sea lions love, Marina and nine other sea lions were set free.
"We gave Marina everything we could to be successful out there,” Westberg told the San Diego Union-Tribune. “We’ve done our part, now it’s up to her. I have all the confidence in the world that she’ll survive out there.”
According to Westberg, experts say Southern California has been experiencing an "unusual mortality event" for sea lion pups.
While experts aren't sure of the cause, Westberg said SeaWorld and other rescue groups are using their rehabilitation efforts as both a chance to save pups like Marina and also learn more about why they're dying.
In 2016, SeaWorld San Diego says it has rescued around 300 marine mammals.