Everland and Caribbean Bay are two of the largest theme parks and water parks in Korea. You have to visit them if you're traveling to Korea!
The best way to visit Everland and Caribbean way is by taking a shuttle bus with a direct route. It's only 6,000 KRW for one-way (12,000 KRW round trip)!
Public transportation can take too long, get crowded, and be super tiring. You're going to be spending a lot of energy at the theme park, so it's way more convenient and comfortable to take a shuttle bus back to Seoul.