Kwon Pyong, a Chinese critic of his country’s ruling Communist Party, already had a reputation for boldness.

Seven years ago he posted a photo of himself on Twitter in a T-shirt that referred to Xi Jinping, China’s authoritarian leader, as “Xitler.” This, and a spate of similarly provocative antigovernment comments from Mr. Kwon, who lived in the northeastern Chinese city of Yanbian, led to a charge of inciting subversion and a stint in prison.

Last week, Mr. Kwon, 35, made another bold move, according to one South Korean human rights activist: He fled China on a personal watercraft, crossing about 200 miles of ocean to reach South Korea, where he had long hoped to seek asylum.

“He was aware of the risk he was taking,” the activist, Lee Dae-seon, who has known Mr. Kwon for years, said on Wednesday. He said Mr. Kwon had told him he was coming, and that they had stayed in contact since the South Korean authorities took him into custody.

The South Korean Coast Guard has confirmed that on Aug. 16 it found a man stranded with a Jet Ski-type vehicle on a mud flat off the country’s west coast, near the city of Incheon. In a statement, it said the man, whom it did not identify, had been detained Sunday on suspicion of entering South Korea illegally by sea from China.

He had set off from the Shandong Peninsula with a helmet, a life jacket, a telescope and a compass, according to the Coast Guard. He also had five containers of fuel, which he’d tied to the watercraft and used to keep the tank filled during the 14-hour journey, the Coast Guard said.

Matt Ran, an engineer from China who lives in New York City and has known Mr. Kwon since 2016, said those details matched plans for escaping China that his friend had shared with him years ago, before the start of the Covid pandemic.

“He felt depressed living in China due to the autocracy and lack of freedom of speech,” said Mr. Ran, 36, who met Mr. Kwon on an online forum about Chinese history. He said they had yet to meet in person, but he called Mr. Kwon a close friend with a “sunny” personality who had taught him to make the “best cold noodles.”

Mr. Kwon, who is of Korean descent, graduated from Iowa State University in 2014 with a degree in aerospace engineering. His Chinese name is Quan Ping, but he preferred to use his Korean name online. Mr. Ran said he “wanted to be a great entrepreneur” and “cared much about China’s democratization.”

Mr. Kwon disappeared into Chinese police custody in September 2016, soon after posting the photo of himself in the shirt that likened Mr. Xi to Hitler. “Let’s work together and topple this invisible wall,” Mr. Kwon wrote in that post. In his Twitter profile, he described himself as a “perpetual student, citizen, dedicated to overturning communism.”

He went on trial for inciting subversion in February 2017 and was sentenced to 18 months in prison, according to Mr. Lee, the Korean activist. The charge was based on 70 or more comments, images and video that Mr. Kwon had shared on social media, his Chinese lawyers said at the time of his trial.

Mr. Kwon was released from prison in March 2018, but the authorities continued to monitor him and barred him from leaving China, Mr. Lee said. He said Mr. Kwon had contacted him in 2019 through connections to other human rights activists, expressing interest in seeking asylum in South Korea.

This month, Mr. Lee said, he received a message from Mr. Kwon after years of silence, saying that he was coming to South Korea.

A South Korean lawyers’ organization, Advocates for Public Interest Law, said that Mr. Kwon had applied for asylum and that it had been asked to represent him in that process. Kim Joo-gwang, a lawyer assigned to Mr. Kwon’s case, declined to comment, saying that he was still reviewing the matter. Efforts to reach Mr. Kwon directly were not successful.

Mr. Kwon’s bid for asylum is far from assured. In recent years, South Korea has granted asylum to fewer than 200 of the more than 10,000 people who have applied for it each year, according to data from the Justice Ministry.