Here's one for the copy book (from pg. 702 of the Modern Library edition of Sodom and Gomorrah)(It's not about the sea):
"At the sound of these words, uttered as we were entering the station of Parville, so far from Combray and Montjouvain, so long after the death of Vinteuil, an image stirred in my heart, an image which I had kept in reserve for so many years that even if I had been able to guess, when I stored it up long ago, that it had a noxious power, I should have supposed that in the course of time it had entirely lost it; preserved alive in the depths of my being--like Orestes whose death the gods had prevented in order that, on the appointed day, he might return to his native land to avenge the murder of Agamemnon--as a punishment, as a retribution (who knows?) for my having allowed my grandmother to die; perhaps rising up suddenly from the dark depths in which it seemed forever buried, and striking like an Avenger, in order to inaugurate for a new and terrible and only too well-merited existence, perhaps also to make dazzlingly clear to my eyes the fatal consequences which evil actions eternally engender, not only for those who have done no more, or thought that they were doing no more, than look at a curious and entertaining spectacle, as I, alas,had done on that afternoon long ago at Montjouvain, concealed behind a bush where (as when I had complacently listened to the account of Swann's love affairs) I had perilously allowed to open up within me the fatal and inevitably painful road of Knowledge."
As I understand the novel so far, this is the key moment of the story--but to be effective it requires your knowledge of a scene from Swann's Way--nearly 2000 pages earlier.
So, spoiler alert. If only I hadn't been so stupid as to assume again that the plot was unimportant in a book of this kind, I wouldn't have spoiled for myself Albertine's eventual marriage to Saint-Loup by poking around on Wikipedia. The very last line of Sodom and Gomorrah has Marcel telling his mother that he's going to marry her. Well, at least I was able to appreciate the irony on my first read rather than later. (EDIT: I'm actually incorrect here about the events in the novel--so this is actually not a spoiler at all.)