Deep Ocean

The floor drops away. A deep ocean holds far more water above the gravel than the ordinary sea, and that depth is what the game checks before it builds the one structure worth diving for.

Ocean monuments generate in the deep variants and nowhere else: prismarine, sponge rooms, a block of gold in the middle and elder guardians that hand out mining fatigue for coming near. Guardians patrol the outside. Cod, squid, dolphins and drowned live in the water column above.

Temperature 0.5, humidity 0.5, mild and temperate, with rainfall rather than snow at the surface. minecraft:deep_ocean is common, came in with 1.7.2, and 1.21 still sinks it under every large sea.

It is Overworld sea, simply deeper. Ocean, Cold Ocean and Lukewarm Ocean all fall away into it, and that drop is the only change you have to notice.

Sail out until the seabed drops far below; the deeper variants are where monuments appear.

Common, so distance is not what stops you, depth is. A conduit or a potion of water breathing comes before the dive, because the floor sits well past what one lungful will reach. No portal is needed, and you cross one while travelling for reasons of your own soon enough: those usual neighbours run into it nearly everywhere the shelf ends.

The same open-water animals as the shallow ocean, plus the guardian. Cod fill their slot in shoals of three to six, squid and dolphins take 14 percent of an attempt each, and the drowned sits at about 1 in 100 with its trident and its nautilus shells.

The guardian is the difference. It fires a charging laser beam and inflicts Mining Fatigue on anything near an ocean monument, which is exactly where the gold in this biome is. A dolphin fed raw cod will lead you towards the nearest wreck or ruin treasure and speeds up your swim while it does, and the squid on the way down drop the ink sacs that black dye needs. The numbers behind that: dolphin is 14% of the larger water life, one to two at a time, then cod all and squid 14%. A squid drops ink sacs, used for black dye and for books and quill, cod leave cod and bone meal behind, and a dolphin fed raw cod grants a speed boost while swimming, which is worth more down here than the meal itself. The drowned is a swimming zombie that will wield a trident given one, so keep an eye behind you when the guardian has your attention. The fish themselves keep to the shoal; watch the guardian instead, and a cod, for example, will not help you.

Squid

Cod

Dolphin

Drowned

Guardian

The same three materials as the shallow sea, met in the opposite order: Kelp first because it reaches nearly to the surface, then Seagrass on the floor, then the Gravel under it once you have the breathing sorted out.

Bring empty slots for kelp, and bring a door or a conduit: the drop here is deep enough that the trip down and back costs more air than the harvest does. The seagrass is planted with its own rule for the deep, thicker than the shallow version because there is more floor to cover.

The list is short, but a deep ocean is not made for shopping: it is made for the monument standing on the floor of it.

Gravel

Seagrass

Kelp

An ocean monument is the reason to sail out this far. The prismarine temple sits on the seabed guarded by Elder Guardians, hiding blocks of gold in its inner rooms, and it generates in only four biomes in this list because it needs water this deep to form.

Around it lie the smaller finds. Ocean ruins, sunken stone structures with loot chests and the maps to buried treasure, turn up in nine biomes on this site; shipwrecks, whose chests carry supplies, emeralds and maps to treasure, reach eleven. All three can appear within sight of each other on one deep floor.

The depth is the whole problem: the seabed is far below, the guardians shoot on approach, and mining fatigue lands on you before you reach the gold. Beneath that floor the world carries on as it does on land, with mineshaft tunnels, dungeon rooms opening off them and the rare stronghold deeper down. Of the 2 wrecks and the monument together, the temple concealing hidden gold offers the greatest challenges, while the ruins and the vessel hold the smaller rewards; exploring all of it is what deep water generation reaches for. Among these three, the monument is able to hold you off longest and is thoroughly the hardest, and its gold is well worth a trip of its own.

Forty eight seagrass tries per chunk, the same rate an ordinary ocean gets, which is the point: depth changes what generates on the floor far less than it changes what swims above it. Kelp comes from noise. Of the 48 passes here, exactly one belongs to this biome alone.

Depth changes the mobs and not their variants: the deep ocean is outside both temperature lists, so nothing about a squid, a cod or a drowned down here differs from the shallow sea.

The map outline rule applies, and the monument does the rest.

The monument is the reason to come, and it is one of only four biomes on this site that can hold one. It lays a guardian spawn list over the whole building and blocks the axolotls and the underground water creatures that would otherwise turn up.

Shipwrecks, cold ocean ruins and an ocean portal fill the rest of the floor.