I have heard of Great White Sharks being spotted near Cheju and the southern tip of the peninsula but never up near Incheon:

South Korea’s coast guard is strengthening its patrols around beaches, helping establish a reporting system and setting up a plan to evacuate people immediately at the sight of a shark after two were found near Incheon, according to a coast guard spokeswoman.

A dead 15-foot shark was caught in a net Saturday by a trawler, and a vacationer found a 1-ton, 18-foot white shark on the beach near Yongyu Island on Sunday. The coast guard said the second shark was alive when it washed ashore but died after struggling for an hour.

The Korea Herald reported it was the first time a great white has been found ashore in South Korea.

“Shark is not a fish group the international community tends to protect,” said Park Jae-young, South Korea coast guard spokeswoman. “If a shark poses a threat to a human, it would be killed instantly. We are ready to kill them when necessary.”  [Stars & Stripes]

Read more at the link.  These sharks that were caught are actually of average length and small in weight.  According to National Geographic the average Great White Shark is 15 feet in length but can grow past 20 feet and weight up to 5000 pounds.  The shark found in Korea weighed one ton or 2,000 pounds.  So it really wasn’t that big of shark.  Plus there is a reason these big sharks aren’t normally found around Incheon because of the massive mud flats there that ultimately caused this