Ocean

The ocean is a common aquatic biome consisting predominantly of water. Oceans are the largest biome by area, covering anywhere around 25-33% of the surface area of the Overworld when factoring in all of its variants, and consists of a deep body of water with the surface at an altitude of Y=63 by default, which is counted as the "sea level" of the world.

Description
Oceans are vast expanses of water, going from sea level all the way down to the ocean floor, which is at a depth of roughly Y=45. The seafloor is fairly hilly, featuring multiple peaks and valleys, and is mostly made up of gravel, though some clay, dirt and sand patches may generate near the peaks while sometimes, ravines can generate on the ocean floor, forming underwater trenches, allowing the ocean to reach even deeper depths. Some seafloor peaks may reach high enough to form islands on the surface of the ocean; these islands are typically small beaches surrounding a patch of a common biome, such as forest or plains. The floor is covered with seagrass and kelp, with "forests" of kelp frequently reaching close to, or even touching the water's surface. Aquatic mobs, such as cod, squid, and dolphins, frequently spawn within oceans. Shipwrecks and underwater ruins frequently generate within oceans and all its variants, and generate more often than other structures in other biomes while the deeper oceans can occasionally generate ocean monuments.

Survival in ocean biomes themselves is rather impractical due to all the water, most players who happen to spawn in on an ocean usually prefer to make their way to the mainland as fast as possible to obtain resources to sustain themselves, though should one decide to camp on an island in the ocean, they will face a somewhat manageable, if still quite difficult situation. Ocean biomes are notoriously vast, one might not find mainland for hundreds or even thousands of blocks (older versions of Minecraft even feature an unofficial ocean variant colloquially called the "Infinite Oceans", which is infamous for being, as the name says, water for millions of blocks on end). This means that whatever resources found on an island might well be all the player has to work with for the earlier stages of the game should they decide to not venture out or be unable to do so. Depending on the island's size, however, the only hostile mobs that the player consistently might have to worry about are drowned from the ocean itself, which makes nighttime less dangerous, assuming the drowned do not spawn with tridents equipped.

When it comes to surviving on an island, consult the Island Survival article for more info.

Variants
Oceans come in ten different flavors in-game; including the base ocean, eight different normal variants and one unused variant, for a total of ten different ocean biomes.

Deep Ocean
In the Deep Ocean variant, the ocean can exceed 30 blocks in depth, making it twice as deep as the normal ocean. The ground is mainly covered with gravel. Ocean monuments generate in deep oceans, meaning guardians and elder guardians can spawn here. Dolphins, cod, and squid spawn in these oceans as well. Underwater caves and ravines often generate here, forming underwater trenches. Exposed lava in these trenches will occasionally turn into magma blocks, creating whirlpool bubble columns that can potentially drag a player down to their doom, so one must be careful around them.

Frozen Ocean
The Frozen Ocean variant has dark purple water at the surface. Like the cold ocean, it has a gravel seabed, though the water's surface is mostly frozen. Neither seagrass nor kelp generates on the sea floor, leaving it entirely barren and reminiscent of pre-1.13 oceans. Large icebergs made of snow blocks, packed ice, and blue ice frequently generate on the surface and polar bears and strays may spawn on these icebergs. Salmon and squid may spawn here as well, and are the only aquatic mobs (other than drowned) that can.

Similarly to swamps, frozen oceans have varying temperatures across their landscapes, but have a more noticeable effect in that colder patches have snowfall and ice sheets, while warmer patches have rainfall (up to a certain height, similarly to mountains) and unfrozen water. This does not apply to their deep variants, however.

Frozen oceans, along with their deep variants, will never generate directly bordering land. They will always have a 32 block-wide border of cold ocean around them should they generate close to any land. However, this border is very jaggy and broken.

Deep Frozen Ocean
Deep Frozen Ocean variants also contain ocean monuments and a deeper floor than normal oceans. Unlike its shallow counterpart, the surface of the water is not frozen, however, surface is completely frozen $$. Icebergs may still generate and polar bears, strays, and salmon may still spawn.

Cold Ocean
The Cold Ocean variant has dark indigo water at the surface. Like regular oceans and frozen oceans, its floor is made up of gravel, although there are scarce patches of dirt, sand and clay on underwater peaks. Salmon, cod, dolphins, squid and drowned are able to spawn in cold ocean biomes, alongside all the other common hostile mobs.

Deep Cold Ocean
The Deep Cold Ocean variant is similar to cold ocean biomes, but twice as deep. Like other deep oceans, ocean monuments are able to generate here, which contain guardians, elder guardians, prismarine and sponges.

Lukewarm Ocean
The Lukewarm Ocean variant has light teal water at the surface. Its floor is made of sand with the occasional dirt, gravel and clay and it contains kelp and seagrass. Underwater ruins that generate in lukewarm and warm oceans are made of sandy materials, as opposed to the stony materials that make up ruins in the other oceans. Cod, tropical fish and pufferfish may spawn here, alongside the other aquatic mobs.

Deep Lukewarm Ocean
The Deep Lukewarm Ocean variant is similar to lukewarm ocean biomes, but twice as deep. Since they are a deep ocean variant, they can generate ocean monuments, resulting in the spawning of guardians, elder guardians, prismarine and sponges.

Warm Ocean
The Warm Ocean variant has light green water at the surface. Like the lukewarm ocean, it has a floor made of sand, and like all except frozen oceans, it is populated with seagrass. Warm oceans feature large coral reefs and numerous sea pickles, though kelp does not naturally generate here. Tropical fish and pufferfish may spawn here, like in lukewarm oceans, though cod does not spawn.

Like frozen oceans, warm oceans never generate directly touching the land. They will always have a 32 block-wide border of lukewarm ocean around them should they generate close to any land. However, this border is very jaggy and broken.

Deep Warm Ocean
The Deep Warm Ocean variant is similar to warm ocean biomes, but without coral reefs nor sea pickles, and twice as deep. Since they are a deep ocean variant, they can generate ocean monuments, resulting in the spawning of guardians, elder guardians, prismarine, and sponges. This biome does not naturally generate, though it can be generated using a Customized Superflat world or the appropriate Buffet world options.