I am a sailor, and I will board the ship in Jeju Island. Do I need a visa to fly from Guangzhou to Incheon Airport and then to Jeju Island?

Yes, lz is an international flight from Incheon Airport to South China, which doesn't meet the requirement of visa-free transit for citizens.

If you fly directly to Jeju for tourism purposes, you can enter without a visa, but because you are boarding a ship, you may not be able to enter without a visa, and you need to apply for a transit visa. For details, please consult the Korean Embassy in China.