Apparently there were times when they observed whales and huge sea monsters that might have attempted to mar or break up their barges. There would have been no reason to mention this statement unless the Jaredites had noted when they surfaced and opened their hatches that these monsters of the sea were in the vicinity. The sperm whale, for example, would be a frightening and no doubt threatening phenomena. And the blue whale is the largest creature on earth. It weighs a hundred tons and attains a length of ninety feet.
We should also mention that back in Ether 2:23, the Lord told Mahonri not to take any fire with them on this voyage. We cannot help but wonder if this journey was similar to that of Lehi's colony on the Arabian desert when they were required to eat their meat raw lest a fire attract various desert marauders.