Ezekiel expects the “many nations” to contain naval assaulters.1 This is relevant because Ezekiel would not (and textually did not) expect Nebuchadnezzar to engage in any naval attacks.2
Textually, “He” (Nebuchadnezzar, personifying his nation) is distinguished from “they” (the "many nations" which would destroy Tyre). This is relevant because, insofar as “he” is most naturally understood as one of “they,” Nebuchadnezzar's army is to be considered only one of the “many nations.”
“Waves” of “many nations” (v3) implies that they will come in successive bouts. This is relevant because one cannot “bring up many nations against you, as the sea brings up its waves” if the one wave, Babylon, was to finish her off (both coastal and insular).
The LXX (Septuagint) rendering says the “many nations” in Ezekiel 26's prophecy are Nebuchadnezzar's multi-national army. (For example, “[he is] a concourse of very many nations”). This is relevant because the LXX better represents the original prophecy here.
But wait, couldn't it simply be that a later scribe wrongly assumed it was Nebuchadnezzar's army, personally feeling it was the simpler explanation (see) and attempted to clarify it accordingly?1
I. Masoretic Text: “thus says the Lord GOD, 'Behold, I am against you, O Tyre, and I will bring up many nations against you... ” (v7) “Nebuchadnezzar king of Babylon, king of kings, with horses, chariots, cavalry and a great army.”
One might here assume Nebuchadnezzar's “great army” was the "many nations" (a common and easy mistake if one isn't paying attention). Unfortunately, because the Masoretic text could easily leave the reader wondering who the "many nations" are in addition to Nebuchadnezzar's “great army,” a helpful scribe could be excused for attempting to clarify that they are in fact one and the same. Verse 7 in the LXX does just that, specifically saying “he is... many nations”; a textbook-worthy example of what well-intentioned scribes do to smooth out the text for readers.
LXX Text: "Nabuchodonosor king of Babylon from the north: he is a king of kings, with horses, and chariots, and horsemen, and a concourse of very many nations."
II. Masoretic Text: “He will slay your people... they will make a spoil of your riches and a prey of your merchandise, break down your walls and destroy your pleasant houses, and throw your stones and your timbers and your debris into the water.”
The Masoretic text is ostensibly inconsistent here. It mingles "he" and "they" in a way conducive to the ostensible misunderstanding outlined above. Noice that again the LXX comes to the rescue and alleviates what the scribe feels is an obscurity, consistently using "he" for the readers sake.
LXX Text: “he shall slay thy people... he shall prey upon thy power, and plunder thy substance, and shall cast down thy walls, and break down thy pleasant houses: and he shall cast thy stones and thy timber and thy dust into the midst of thy sea.”
To say it was simply Nebuchadnezzar's army that was to attack Tyre would be the simplest explanation of the data. This is relevant because when explaining data, the simplest explanation tends to be best (Occam's razor).
But wait, the simple explanation is only best with all other things being equal. In this case however, the explanation fails with respect to other explanatory virtues.1 Saying that more “nations than Nebuchadnezzar's are prophesied to attack Tyre” is necessary, in light of the three arguments above.