Korea Tourism has come out with a new slick and more adventurous option to reach Nami Island from Gapyeong Station apart from the normal ferry ride from the jetty and you might just want to try it. If given the chance, I will definitely do it!
It is called as Zip Rider / Zip Wire
According to the video, the Gapyeong Tower's height is about 80 metres and the distance from Gapyeong to Nami Island is about 940 metres. The Zip wire ride takes about 1 minute 30 seconds. If you reach the Jara-Nami Skyline, take the lift to 3rd level for Nami Island.
From Seoul Station to Gapyeong Station by subway
1 hour 44 minutes
2 transfers - Hoegi Station (subway) and Sangbong Station (normal train)
1,800 won by cash, 1,900 won by T-money
From Gapyeong Station to Nami Island Ferry
Gapyeong Station (Jungang Line), Exit 1
From here, walk straight to the bus station about 200 metres then take bus number 33-5 to Nami Island Ferry. Or you may take a taxi.