I took this video and the shots below on my iPhone on a recent trip to Yangsuri. I wish I had had my Nikon with me, but the iPhone got the job done all right.
Getting there: Just take Line 1 from either Yongsan or Oksu stations, and head way out east. It’s the second stop after you pass through a long tunnel. Get off at Yangsu Station and head down the hill, towards the water.