Sea Mine

Any ship can be a minesweeper. Once.

Ah, naval mines. Just the thing to catch unwary vessels.

The use of these Naval Weapons is still legal unlike the anti-personnel land-based version, but you are required to notify people of their use and the rough location of them, so civilian shipping can stay out of the way.

Naval mines in fiction are always portrayed as large metal spheres covered with small spike-like detonators which cause the mine to explode on contact with a ship (or any unfortunate individual). This is based on the appearance of early (World War I-era) naval mines. Most modern ones look rather different. Some are self-contained launch tubes for a homing torpedo that launches when it detects the sound of a ship or submarine's propellers (and is smart enough to distinguish between the two, or even different classes and sizes of ship, and may be set to attack either or both). Others are modified aircraft bombs dropped in shallow waters to lie on the sea bed, with sensor circuitry that detonates when they detect the change in magnetic fields created by the ship's metal hull. The horned type, however, remains a favored weapon for shallow waters and low budgets, and like the Cartoon Bomb is easily recognized by the viewing public.

Examples of Sea Mine include:

Anime and Manga

The Digimon Chikurimon is one.

Film

At the beginning of For Your Eyes Only, a British spy trawler accidentally fishes in a sea mine and sinks. It seems like a freak accident, but it was actually planted by The Dragon, who later blows up another on dry land to effect his escape from 007.
- In Moonraker, Bond's Amazon boat has a number of these in its arsenal, which Bond releases to blow up a pursuing boat.
Finding Nemo
- Those would have had to be Allied ASW mines as the Japanese would not have been able to lay mines and the Allies had no motive to lay any other type. By an example of Hollywood Tactics the submarine seems to have been an Allied sub (if I remember; it's been a long time since I saw the movie). While of course mines wouldn't know the difference, Allied subs would not be submerged near Australia. They would be running on the surface, with an escort to prevent friendly fire, too high to be hit by ASW mines.
Hot Fuzz, except it's not underwater when we see it, it's lying around in some dude's garage.
- Though the dude hanging onto it claims that it's a dud. It's not.
The Heroes of Telemark have to fend off a mine with a big pole while escaping from Norway.
The protagonists of The City of Lost Children have to navigate through a bunch of these to get to the lair of the resident Mad Scientist.

Live-Action TV

The 1970's Australian series "Patrol Boat" had an episode where a Japanese WW 2 sea mine had drifted inshore, but blowing it up would mean destroying some historical Aborginal cave paintings. Forget how it was resolved.
One of these washed up in the lagoon in an episode of Gilligan's Island.
Done in the first series of Sea Patrol, when a mine washes up on a beach after a cyclone. While it is unstable, the only real danger comes from a pair of Too Dumb to Live preteen boys who find the mine, don't report it straightaway. One even runs towards it when the bomb disposal unit are about to destroy it to save a tortoise.

Radio

The Goon Show: "The Dreaded Batter Pudding Hurler".

Eccles: MINE AHEAD! Dirty big mine ahead!
Bloodnok: Mi—? (footsteps running into the distance, splash)
Neddie Seagoon: Funny, he wasn't dressed for swimming.
Eccles: Wait, fellas, there's no need to worry about the mine — it's one of ours!
(KABOOM)

Tabletop Games

Widely used in Harpoon
BattleTech allows for the deployment and use of minefields as part of its advanced (i.e., not strictly tournament-standard) rules. This includes explicitly sea-based conventional, inferno, or command-detonated minefields; inferno mines only work on the surface, the other two can be placed at any given depth.

Video Games

Minesweeper.
Can be deployed in Harpoon
The submarine Shoot'Em Up In the Hunt has LOADS of these in the first stage and the final stage.
In Steel Saviour, the first Mini Boss drops these near you, which explode in a large Sphere of Destruction.
The Pokémon Koffing's appearance is based on a floating naval mine. Since it learns Self-Destruct and Explosion, this is fitting.
Sly Cooper 3 features these as a hazard in some stages, as smallish items (often with their own inner tubes for flotation). The last chapter notes your approach to magnetic mines in a ship. The Cooper Gang avoided Critical Research Failure and Critical Existence Failure by bringing the ship from the prior chapter. Made primarily of wood.
The logo for Relic Entertainment.
Crysis had them for some reason.
Appeared in one mission in Grand Theft Auto Chinatown Wars.
You can find them scattered on a dry lakebed in Borderlands: The Secret Armory of General Knoxx. If you drive a car too close, they will explode and kill you.
Common obstacles in the first two Crash Bandicoot sequels.
2/3 of the underwater stages in Alundra 2 feature these.
The SNES platformer Claymates had a few of these floating around some of the longer underwater sections.
Duke Nukem 3D features small mines in the "Toxic Dump" level.
Sub Culture had whole fields of this, sometimes made extremely dense for some missions.
The player's ship has to navigate minefields every now and then in the Naval Ops series. You can also lay mines, but they don't do very much damage and getting another ship to run into them is iffy at best.
There were a few in the Pirate's Fortress in The Legend of Zelda Majoras Mask.
Kingdom of Loathing has mine crabs, hermit crabs that claimed naval mines as their homes. See Demonic Spiders.
Bug!! had these in Quaria. They blew up if you touched them, of course.
Featured in the seventh story mission of Jaws Unleashed. Just touching the chains which they're attached to would make them explode.
The first Venice level in Tomb Raider II has a group of sea mines in by the level exit. Naturally, you couldn't drive your boat through the area unless you wanted to suicide but the trick to clearing the mines is to accelerate to the mines and jump out of the boat before impact.
In Banjo-Tooie, the Submarine Challenge minigame involves destroying a bunch of these (called Shrapnel) for points.
Lots of them show up in GoldenEye: Rogue Agent in a cutscene when you arrive at Crab Key.
Shows up in the game Super Mario World.

Web Comics

Newshounds featured a dolphin whose former job was to find these. He wasn't very good at it. Favourite line: 'Boom boom boom go the ocean zits!'

Western Animation

On Centurions, Max Ray's Depth Charger weapon system was equipped with one called a "hydromine". He once used several of them to stop a tsunami from occurring when a massive meteorite landed in the ocean.
The unfortunately-named "Stink 'n' sink" mines from Avatar: The Last Airbender, invented by Hakoda.
On the Looney Tunes short "Porky's Snooze Reel", a jellyfish swallows a mine whole, despite the narrator's warnings, and is blown into jars of Jell-O.

Other Media

Naval mines were used in the 1998 MBX The Battle of Brunei. The Malaysian naval commander gave the Refuge in Audacity order that the minefields be regularly chummed.
Dutch comedy Audio series Ome Henk has the titular Uncle Henk berate his Director for misinforming his nephew Jantje for calling a Seamine a Buoy. His nephew responds by throwing it away, causing a GINOOOOOOOORMOUS explosion. As per usual.
It's widely accepted that a Navy Mine sunk the Hospital Ship The HMHS Britannic. The 2000 Made for TV Movie however played with the alternate theory of a submarine torpedo but in the end it was the work of a saboteur.
In Operation Starvation American Superfortress bombers were detailed to dropping mines of an assortment of types around the Japanese home islands.