The fishermen were sounding the bay's depth to make sure the boat could pass safely.

sounding 是动词 sound 的现在分词形式。sound 在这里用作动词，意思是"测量深度"，特别指用声呐或铅锤测量水域的深度。这个词义来源于航海术语，通过发出声音信号并测量回声时间来测定水深。