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.
之所以我们必须在冬天工作， 是因为他们没有在夏天和春天工作足够的资金。 如果在春夏工作， 那么他们就需要船和潜水艇来辅助工作。