The “we” in this verse appears to refer to Mormon and his time. For Mormon, there was no record of any further return, nor any indication in the subsequent years of what happened. The process of sailing on the ocean was not complete foreign to Mormon, for he knew that one of the possibilities of sea travel was failure. Thus they “suppose[d] that they were drowned in the depths of the sea.” In spite of the fact that no other ship returned, still one more left.
While the loss of the ship at sea is entirely possible, it is more probable that they reached a coastal landfall to the north, and either merged with the people in that area, or were killed by them.