You can also travel to China, Japan and Russia by ferry. Ferry routes are less expensive but slower than air. To use the ferries, you require your passport:

Ferry to/from China

Inquiries: +82-32-880-3300, +82-1599-5985 (Korean, Chinese)
Ferry schedule: Please check the official website to confirm the details. (Korean only) https://goo.gl/tkWFH8

Departure                 Approx. Travel Time         Arrival

Dandong                        16hrs                                Incheon

Dalian                            17hrs                                Incheon

Yingkou                         24hrs                                Incheon

Qinhuangdao                 23hrs                                Incheon

Yantai                            15hrs                                Incheon

Shi Dao                          15hrs                                Incheon

Lianyungang                  24hrs                                Incheon

Weihai                           13hrs                                Incheon

Qingdao                         16hrs                                Incheon

Tianjin                           24hrs                                Incheon

Weihai                           12hrs                                Pyeongtaek

Lianyungang                 20hrs                                Pyeongtaek

Rizhao                           16hrs                                Pyeongtaek

Qingdao                        17hrs                                Pyeongtaek

Ferry to/from Japan

Busan Port International Passenger Terminal Ferry schedule: Please check the official website to confirm the details. (Korean only) https://goo.gl/rzNSRK

Departure                 Approx. Travel Time              Arrival

Fukuoka                             3hrs                                Busan

Shimonoseki                     11hrs                               Busan 

Izuhara                               1hrs                                Busan

Osaka                               19hrs                                Busan

Ferry to/from Russia

Donghae Port International Ferry Terminal
Ferry schedule: Please check the official website to confirm the details.

Departure                 Approx. Travel Time              Arrival

Vladivostok                    20hrs                                Donghae

For additional ferry information, please visit https://goo.gl/h7GVON

Ferry schedules are subject to change depending on the weather and other unforeseen conditions, so you should get details from the official websites before proceeding.

For more information about ferries, please see the website at https://goo.gl/Lh6LHd