An incomplete list of ocean horror.
Underwater (2020): Kristen Stewart and a small cast of rapidly dwindling fish food discover that their deep see mining operation may have delved too deeply and greedily into the earth, and some things buried should be left to sleep. Relatively big budget creature feature which suffers from sitting on the shelf and being tinkered with for a couple of years before releasing in COVID. Some odd editing decisions and not a lot of character work, but it looks good, it SOUNDS great (more important than you might think for ocean horror), at least two memorable deaths which push the PG-13 rating to the limit, and a whopper of a last act reveal. Also, 94 minutes long, which is a strong endorsement, imo.
Leviathan (1989): Peter Weller, of Robocop, stars in this soggy mish mash of Alien and The Thing. A team of deep sea miners stumble upon the wreck of a Soviet ship and wind up salvaging more than they bargained for out of the Captain's safe. Creature effects by Stan Winston, but lower your expectations for the finale, as it's definitely not his best work. Still, fun in a goopy, cheesy way, buoyed by a winning cast.
The Abyss (1989): Pretty much the same set up as Leviathan, but executed by James Cameron instead of George P. Cosmatos. I love Cobra as much as the next guy, but Cameron is obviously the superior director. Takes a turn towards navel gazing sentimentality towards the end, which could be a pro or a con depending on what youre looking to get out of the experience.
Deep Star Six (1989): I've not actually seen this one, but it completes the trifecta of 1989 ocean-based science-fiction horror movies, so it needed to be included.
Deep Rising (1998): Treat Williams is hired to ferry a team of mercenaries to a rendezvous point with a luxury ocean liner, where they intend on looting and scuttling the ship. Unfortunately for everyone, bobbit worms' bigger, grosser cousins show up start gorging themselves. Directed by Stephen Sommers, right before launching into the Mummy the following year. Impressive (for 98) CGI, solid R-rating, and another winning cast (Treat Williams, Famke Janssen, Anthony Heald, Kevin J. O'Connor, Djimon Hounsou, and Jason Flemyng).
The Poseidon Adventure (1972): Breaking from my monster movie convention to recommend this, which is a disaster film with nothing supernatural about it. However, I think there are sections which are tense enough to qualify as horror-adjacent, and again, what a cast, man. If you can't tell, I'll put up with a lot of crap if I find the actors compelling in some way. Unlike Treat Williams though, I shouldn't have to justify to you the enjoyment of watching Gene Hackman, Ernest Borgnine, Shelley Winters, Red Buttons, Roddy Macdowell, Leslie Nielsen, AND MORE navigate an ocean liner which has been capsized by a rogue wave. With the exception of Jaws and The Abyss (arguably), this is the best movie on this list. Light recommend for the 2006 remake, which is nowhere near as good imo, but does take advantage of the technology of the time to emphasize the disaster segments. And I'm a sucker for Kurt Russell.
Below (2002): WW2-set submarine ghost story starring a slew of character actors, and a very early role for Zach Galifianakis. I've seen this before, multiple times, but probably not in the past 20 years. It was a staple on the IFC channel right around the time my dad sprung for the expensive cable package. I remember it being an effective, if somewhat slight, spook-em-up story, bolstered by the unique setting. Written by Darren Aronofsky (Requiem for a Dream, The Whale, Black Swan, etc.) and directed by David Twohy (the Riddick movies, which is an endorsement from me, but may not be for others).
Jaws (1975): it's THE shark movie. Nothing more to say, if you haven't seen Jaws and you're curious about ocean-based horror, this is where you have to start.
Beast of War (2025): in WW2 Australia, a troop of ANZACs are stranded when their carrier is sunk, and they're left adrift among the wreckage. Then, the shark arrives. Gnarly effects, more competent writing and acting than this genre usually pulls off, and moody cinematography all elevate this out of the depths of "shark movie trash" and into "enjoyable B-movie" shallows.
The Meg 1 and 2: Jason Statham and a diverse cast of stars from major non-US film markets have to take down giant sharks and other prehistoric escapees from a primordial undersea trench. It's not good, but they throw enough money at the effects to make the attack scenes fun. Check your brain at the door, it will be a detriment to your enjoyment.
Deep Blue Sea (1999): An Alzheimer's researcher accidentally creates super smart sharks by enlarging their brains. The sharks use their newfound intelligence to pick off the crew of this animal testing facility one by one. Early Stellan Skarsgard role. Also features LL Cool J as the religious and borderline insane facility cook, and Thomas Jane as the shark wrangler protagonist. Aggressively stupid, but all the more fun because of it. Mister Cool J raps over the closing credits with an original song written for and referencing the movie, which is a bold and hilarious decision.
Blood Vessel (2019): Super light recommendation for this one, as it isn't really in line with your request. The maritime setting is mostly incidental here, but it's an underseen indie horror movie that is technically set in the middle of the ocean, so I'm throwing it in. A cast of inexplicably diverse WW2 allies are adrift when a seemingly abandoned German hospital ship approaches them. They board the ship and discover that something nefarious went down on board, related to a couple of strange crates in the hold which are of special interest to Nazi high command. Stars Alyssa Sutherland, who would go on to appear in Evil Dead Rise as the possessed mother character.
I'll add to this if I think of others.