One of the reasons that line about the waves is intriguing is because of what we actually see of the White Walkers and wights in this episode. Looking back at the scene, it does appear as though the ground in front of them isn't just the snowy terrain we're used to seeing Beyond-the-Wall.
When first analysing this a bit closer, I thought it might've been grass: it does appear as though you can see patches being iced over. The implications of that would be we're seeing the future here, after the White Walkers have invaded Westeros, and are well on their way towards the living and the Great War.
Combined with the missing line, however, it adds to the possibility that what we're seeing here is the army actually walking on the frozen water, although that doesn't suggest either way if it's the past or present.
8. The Opening Credits Additions
The opening credits of Game of Thrones are always worth sitting through, given they clue you in to where we're going that week (and just look great), but that's especially the case when major changes have happened or a new location is being introduced.
Dragonstone, then, has a couple of important pieces on the map. One is the return of Dragonstone itself, which hasn't been featured since Season 4, and now has the Targaryen sigil on it. Speaking of sigils, King's Landing remains Baratheon, even though Cersei Lannister has decked out her Throne room with a Lannister lion.
The biggest change, though, is with the addition of Oldtown. Here we get to see the Hightower - ruled by House Hightower, fittingly - which stands in the middle of the city. In a fun bit of mapception, we see an astrolabe in the middle of it, the same spherical objects that appear in the Citadel's library, and as part of the opening credits since the beginning.
The people who make the map envisage it as being made by a mad monk in a tower, keeping track of the events of Westeros and beyond. If they're indeed Maesters, then it means they're putting themselves in the map they've created.
On a final opening credits related note, this is the first time they haven't shown any part of Essos, since Daenerys has finally made it to Westeros.
7. The Word But...
Ned Stark gets referenced in this episode in the scene between Jon and Sansa, with the King in the North telling his sister that their father (well, his uncle) used to say "everything before the word but is horses**t."
It's not the first time we've heard these wise words of Eddard quoted on the show: way back in Season 1, Benjen Stark said the same thing to Tyrion Lannister.
It's not the only reference to his father Jon makes in Dragonstone, as he also reiterates his core belief that "the man who passes the sentence should swing the sword."
Ned's obviously a big influence on Jon in terms of his honour and ideals, though hopefully the latter won't make the same mistakes. There'll be more Ned quotes to come too, as we already heard in the trailer Sansa using his book line: "...The lone wolf dies, but the pack survives."