After them, i.e. after the children of Israel. Note here, the relative is put without an antecedent before it; the antecedent being to be understood and gathered out of the following verse, or out of the course of the story. An observation which is very useful for the understanding of many scriptures. See Poole on "Genesis 3:1".
Related Commentaries of Exodus 14:28
Exodus 14:28
28 And the waters returned, and covered the chariots, and the horsemen, and all the host of Pharaoh that came into the sea after them; there remained not so much as one of them.