The Sea is a Lunar-Stellar element, and our focus in this context is typically on its terror, its immensity, its oppressive darkness and mystery. I've actually found it surprisingly hard to find "cosmic horror, but make it the sea" recommendations - a lot of my development for this was just reading too much of the Scuba Disasters board in various depressions, and growing up near the sea. Although one of my barriers is also I really can't watch a lot of sea disaster content without having to take hyperventilation breaks, so...
The Abyss (1989)
Film | So close, so far. There's far fewer films about the horror of the sea, either its natural and physical horror, or horror of a cosmic and weird kind. Abyss is one of the few. You can feel two wolves within this film, one hoping to be "2001: A Space Odyssey but Wet", the other yearning for the more Speilbergian Close Encounters/E.T. vision of the alien - a source of wonder and (comforting) moral judgement on man - a sort of contemporary take on angels. Anytime the Abyss is doing the former, it is strongly recommended - a film which communicates well the practical difficulties that humans face in the depths, from pressure sickness, to the cold, to panic, to the dark. It just can't resist the lure of the 1990s Hollywood feelgood big-budget disaster-movie. That said, I do still recommend it - because there are far too many films about the sea that don't even consider its cthonic qualities at all.
The Last Dive (2002)
Book | Biography of a father-and-son SCUBA diving team who met with tragedy exploring a mysterious U-Boat. A better read than Deep Descent, but does not quite hit the spot for our purposes; for completionists only. I basically think the kinds of people who are sufficiently into SCUBA to want to write about SCUBA are not natural poets, if that makes sense; it's a retelling of a fascinating true life story, but not art.
The Lighthouse (2019)
Film | Unseen. Sea horror by the director of the VVitch? Sign me right up.
The Perfect Storm (1997)
Book | Unread; non-fiction; trawlermen in a deadly storm
Shadow Divers (2004)
Book | Unread; non-fiction; discovery of a U-Boat
Film | Psychological thriller about a mystery alien presence at the bottom of the sea. It passed an afternoon, and the performances were good, but it very much wimped out on both the sci fi and the potential for weird (instead of just horrible). About on par with the Abyss, but that film has far better sea horror content. I wouldn't look it out
Deep Descent (2001)
Book | Non-fiction account of explorers on the shipwreck known as the "Everest of SCUBA diving". An interesting story, let down by an unflamboyant writing style; not recommended for this project unless you are a completionist. There are so many spooky Andrea Doria stories and somehow, this doesn't hit the spot in telling them.
✪ Moby Dick (1851)
Book | The great classic of the cosmic sea, and a fantastic read. A wet, sepulchral mood throughout, oppressive both verbally and in its imagery. I've failed at pulling quotes out, because it doesn't seem to work out of context: you need that great weight of words bearing down on you like the empty sea. It's hard to know whether to recommend it or not, because in many ways it's not specifically for us; but on the other hand, there's really no better way to meditate on the ocean than this.
For Those In Peril (2013)
Film | Unseen. Wright made Arcadia, which is very very good; and I'm on the look out for folk-horror-on-sea, a genre which is inexplicably under-explored.
Webpage | Less than 10% of the sea has been explored. If you're not yet profoundly terrified by this, spend some time on this reddit dedicated to images which will encourage a phobia of the vast and fathomless cold. The subreddit has, lamentably, gone down in quality; but you should use the links on the side to find the "exemplar" posts and start with them. Browse on old.reddit rather than reddit or new.reddit for best impact.