The curses listed in this chapter of rebuke (Tokhekha) end with a verse that appears to stand on its own: "And the Lord will bring you back to Egypt in ships" (28:68). This is not a general threat of exile, rather it seems to be a specific threat to nullify the children of Israel's status as the Chosen People if they do not keep their part of the Covenant and to return them to the pre-Exodus days of slavery in Egypt.
With the backdrop of the Exodus, and in the narrow context of verse 68, the word 'ships', could allude to slave-bearing ships. Yet in the broader context of this verse, it would seem that the ships are a contrast to the Splitting of the Reed Sea (Yam Suf). This miraculous event was the culmination of the Exodus when the children of Israel walked through the dry land and were finally released from slavery. This verse emphasizes that if we will be punished for our sins and sent back to Egypt, there will not be a repeat of the miraculous splitting of the sea, but rather we will be sent back by ships – slave-bearing ships.
The contrast between walking through the dry land with our heads held up high and the water forming walls to our right and left and the return to Egypt in ships is an expression of the contrast between redemption and slavery.
Based on this explanation, we may now understand why the Torah emphasized that the return to Egypt will be "through the way about which I had said to you, 'You will never see it again.' "
This verse teaches us that this was a conditional promise, and violating its terms will lead to its annulment. God, who brought us out of Egypt, will Himself lead us back, specifically along the same way that He had taken us out, in order to emphasize the significance of this return.