The ancient Yunmeng may have been deep, but the modern Dongting Lake definitely is not.
West Dongting Lake covers an area of 345 square kilometers and has a depth of 35-36 meters; South Dongting Lake covers an area of 917 square kilometers with a depth of 34-35 meters; East Dongting Lake spans 1,478 square kilometers with a depth of 33-34 meters.
The entire Dongting water system, including numerous river tributaries, has an average depth of only 6-7 meters. Even with the continuous heavy rainfall causing water levels to rise, it still hadn't reached an unfathomable extent.
The problem was the water area was too vast. With a brood of young jiaos hiding at the lake's bottom, searching with only fourteen people was simply unrealistic.