The wedding of Daemon and Rhaenyra is the first wedding done in Valyrian tradition in both the books and the HBO shows. The vows and traditions were created by the writers of the show, using what knowledge is given about Valyria.
The episode title refers to the island of Driftmark, location of the ancestral seat of House Velaryon. Castle Driftmark, the original seat on the island, was built of dark stone and prone to dampness and flooding. Lord Corlys therefore had a new castle, High Tide, built on the opposite side of the island, where there is more exposure to sunlight. High Tide is constructed from the same lighter-colored stone as the Eyrie.
Helaena's apparent ramblings in fact foreshadow events from the looming Dance of Dragons civil war between Alicent's Greens and Rhaenyra's Blacks. She says, "Hand turns loom; spool of green, spool of black. Dragons of flesh weaving dragons of thread."
This episode has the first mentioning of either faction in the Dance of the Dragons by name, with Rhaenyra referring to the Hightowers and their allies as "the Greens".
In the book Laenor's death is not faked. Qarl Correy actually kills him. There's debate, however, about Qarl's motives: whether he was put up to it by Daemon, or whether it was a lovers spat which turned violent. The show reverses this, with Laenor's death being faked, but Qarl's motives being clear, at least to the audience.