Incheon air port --> Shenzhen Bao'an International Airport ---> (Train or subway) --> Hongkong

Your itinerary:

Seoul - Shenzhen - Hong Kong

Is eligible for 144 hour visa free transit, but under the following two conditions:

1. You can leave Shenzhen to Hong Kong only by ferry from Shenzhen Shekou port by CKS - Chu Kong Passenger transport company and you have to book a ticket through them for this route, before your departure from Seoul. Also, you will have to print out that ticket and have the printout with you as proof of your visa free transit eligibility.

2. Your departure by ferry from Shenzhen can be at least 24 hours and 1 minute after your scheduled arrival by plane. So, you have to stay within Guangdong province where you are not limited only to Shenzhen, but you can travel throughout the province, at least for 24 hours and 1 minute before you are allowed to depart by ferry to Hong Kong.

You cannot use any other mean of transport at the time being.