Final answer:
A synonym for 'fathom' in the context of the provided sentence is 'comprehend,' as it reflects the need to grasp or understand something difficult to perceive.
Step-by-step explanation:
Using the context of the sentence in question, a synonym of the word fathom can be comprehend. The passage describes a situation akin to being at sea in dense fog, indicating a need for understanding or grasping the situation in the absence of physical navigation tools. The word fathom in this context signifies more than measuring depth; it also conveys the idea of understanding or grasping something that is difficult to perceive. Therefore, comprehend serves as an appropriate synonym for fathom in the context of the sentence provided.