Deep Frozen Ocean

Ice on top, monument underneath. The deep frozen ocean is the only water where you may have to mine through the surface to reach a prismarine roof.

Icebergs of packed ice and blue ice stand out of it, polar bears walk them and can stand on more than just ice here, and salmon, squid and drowned swim in the dark below with guardians around the monument. Blue ice cut from those bergs is the fastest travel surface in the game.

It reads 0.5 for both temperature and humidity, a mild temperate climate on paper, but the surface freezes and what falls is snowfall rather than rain. minecraft:deep_frozen_ocean is uncommon, arrived in 1.13, and 1.21 still locks it under ice.

The coldest deep water the Overworld makes. Frozen Ocean and Deep Cold Ocean are what it opens out of, so you come to it over ice rather than over waves.

It is the deepest frozen water, with icebergs above and an ocean monument sometimes lurking beneath the ice.

Uncommon, and awkward rather than distant: the surface is solid, so bring a pickaxe to open a hole or use the gaps the icebergs leave. Polar bears defend that ice and guardians wait underneath it. Watch the ice rather than the water: an ordinary frozen sea turns this deep nowhere else, and the icebergs reveal the change long before the floor does. Its neighbours above are what it soon opens out of.

Salmon fill their slot completely in shoals of up to five, and polar bears fill theirs on the ice above, one or two at a time, passive until you get between a bear and its cubs. Squid hold about a third of the water, one to four at a time, dropping the ink sacs black dye is made from.

Underneath sits the guardian, firing its charging laser beam and inflicting Mining Fatigue on anything that approaches the monument. The drowned is on the list too at roughly 1 in 100, trident and nautilus shells included, and under an ice sheet neither of them is easy to swim away from. Behind it, salmon all and squid a third. A polar bear is passive but defends its cubs aggressively if you stray too close, squid drop ink sacs: black dye on one side, books and quill on the other, and on the peaceful side that is the whole list. The drowned is a swimming zombie that will wield a trident given one, and a meal here is salmon or nothing. Cod give bone meal here as anywhere, and a salmon drops what it gives while the ice holds; a squid, for example, is easier to catch than either. Underwater, everything you meet is used to the cold before you are.

Squid

Salmon

Drowned

Guardian

Polar Bear

Ice, Packed Ice and Blue Ice on top, Gravel far below. It is the only sea that gives you a solid roof and a deep floor at once, and the icebergs are where the blue ice comes from.

Water freezes over on its own up here, so the hole you cut to get down closes behind you, and the fastest way back out is the one you left open with a torch or a door.

Before you build down here, it is worth knowing that the roof over your head grows back on its own.

Ice

Packed Ice

Blue Ice

Gravel

A monument under an ice sheet is as awkward as this game gets. The prismarine temple generates on this floor, in one of only four biomes anywhere in this list with water deep enough, guarded by Elder Guardians and concealing gold blocks in the middle.

Above it float icebergs and blue ice; below it lie ocean ruins, the sunken stone with its chests and its maps to buried treasure, counted in nine biomes altogether, and shipwrecks, present in eleven, holding supplies, treasure maps and emeralds.

Plan the way out before the way in, because the surface freezes over and the gap you dived through may not be there when you come back up. Mineshaft tunnels, with dungeon rooms opening among them and rarely a stronghold, sit beneath the seabed regardless of the ice. The temple keeps its gold hidden, the vessel whose chests hold supplies sits nearby, and exploring either offers loot and challenges under a ceiling of ice. Each of them is able to appear under the same ice sheet, and these three are thoroughly well hidden.

Two kinds of iceberg, and the numbers are far apart: a packed ice berg lands on one chunk in 16, a blue ice berg on one chunk in 200. Blue ice on the floor gets up to 19 tries. Forty nine passes altogether, which is more than an ordinary ocean gets, and all of the extra is ice.

Polar bears may spawn on more than the usual blocks here, exactly as in the shallow frozen ocean, so they can appear on the ice sheet itself rather than only on land.

Cold farm animals and cold frogs are the other two rules on this biome, and the water is drawn on a map outline. Squid, salmon, drowned and the bears are what you meet.

A monument under the ice is the strangest combination on this site: the deepest cold water in the game with a guardian spawn list inside it and a frozen lid over the top.

Cold ocean ruins, shipwrecks and the ocean portal are the rest, and none of them cares that the surface is solid.