The reason why they have to work in winter is because they don't have the money to work in summer and spring, which, if they did that, they would need ships and submersibles to do their work.
그들이 겨울에 작업하는 이유는 봄과 여름에 할 비용이 없기 때문입니다. 만약 그때 한다면, 배와 잠수함들이 필요하기 때문이죠.