This war took a tremendous toll on both sides. We are not told anything more about the disposal of the dead other than “they did cast their dead into the waters of Sidon.” It is possible, but not probable, that this included Nephite dead. We know that the Nephites had a preference for burying their own dead which we have seen in Alma 3:1. The denial of the burial to the Lamanites by casting them in the river to be “buried” in the sea would have been a religious and social insult to the warriors of the Lamanites who had fought against them. This would appear to be a parallel to a situation we have seen before:
Alma 3:1-3
1 And it came to pass that the Nephites who were not slain by the weapons of war, after having buried those who had been slain—now the number of the slain were not numbered, because of the greatness of their number—after they had finished burying their dead they all returned to their lands, and to their houses, and their wives, and their children.
2 Now many women and children had been slain with the sword, and also many of their flocks and their herds; and also many of their fields of grain were destroyed, for they were trodden down by the hosts of men.
3 And now as many of the Lamanites and the Amlicites who had been slain upon the bank of the river Sidon were cast into the waters of Sidon; and behold their bones are in the depths of the sea, and they are many.
In Alma 3 we have two different types of disposal of the dead. We have burial for the Nephites, and dumping in the Sidon for the Lamanites. It would appear that we should read these verses in this chapter with the same understanding of the two different types of disposal of the dead. The dumping in the Sidon was a mode of disrespect, and while Mormon does not make the distinction clear, to him it would have been understood and not worth clarifying.