Nephi said that “in the days that the prophecies of Isaiah shall be fulfilled men shall know of a surety, at the times when they shall come to pass” (2 Nephi 25:7). This prophecy in verse 15 is one of those which will probably not be understood until it comes to pass, but when it does there will be no doubt that it is fulfilled. The identification of “the tongue of the Egyptian sea” and “the river,” which today is uncertain in the minds of biblical scholars, will become certain when the Lord delivers his people. According to Oliver Cowdery, the events in this verse and those in verse 16 would need to occur so that the gatherings could be fully accomplished. Elder Orson Pratt referred to this prophecy as a “literal transaction.”
The same thing, not a spiritual, but a literal transaction, as the Lord smote the tongue of the Egyptian sea in ancient days, and caused his people to go through on a highway in the midst of those mighty waters which stood like walls on each side of the assembly of Israel. So in the latter days he will not only cut off the tongue of the Egyptian sea, but the river in its seven streams will also be divided and men will go through dryshod. This is the testimony of the prophets concerning the events that are to take place when the times of the Gentiles are fulfilled. [JD, 14:66]