This guide is a simplified overview of all the changes in Update Aquatic (Java Edition 1.13.x releases), which assumes at least some familiarity with the World of Color Update (Java Edition 1.12.x releases).
Players receive the Dolphin's Grace status effect when sprint-swimming near dolphins. This effect allows players to swim significantly faster underwater.
During the day, drowned stay on the ground underwater, ignoring players on land no matter how close they may get, they only become hostile during daytime if the player touches water.
Spawn at night and during thunderstorms high above players who are above sea level, have no obstructions above them, and have not entered a bed for at least three ingame days.
Phantoms are the fastest mob in the Overworld. Players are unable to outrun them. A good strategy versus phantoms is to obstruct their path, since they fly up if they collide with any block during an attack.
Cats hiss at phantoms that are currently attacking players, and phantoms try to stay at least 16 blocks away from them.
Phantoms do not spawn if the player has a solid block above them, meaning they do not spawn around players underground or within enclosed structures. Additionally, they do not spawn if the player is below sea level.
Sleeping through the night is not actually required. The statistic that controls their spawning is reset when the player leaves a bed, so simply entering and then leaving a bed is enough.
When worn, provides ten seconds of Water Breathing while the player is out of water or in a bubble column, which starts counting down when the player goes underwater.
A partially frozen area of ocean, with large, broken up sheets of ice covering the water. This variant of ocean has a gravel seabed and is home to kelp, seagrass, squid, salmon, strays, and polar bears.
This ocean variant can be recognized by its light teal water and sandy seabed. It is home to kelp, seagrass, pufferfish, dolphins, tropical fish, and cod.
An ocean variant with bright teal water at the surface. It can contain coral reefs, sea pickles, seagrass, dolphins, tropical fish and pufferfish. It does not have a deep variant.
The physics for water have also been changed, and the way the player interacts with water has also changed.
Swimming
There is now an actual swimming animation used by the player while underwater, which can be activated by sprinting underwater. This also allows the player to move in any direction they are looking including down. Players who cannot sprint, i.e. are too hungry to sprint, cannot swim. Crouching underwater also allows the player to descend at a faster rate.
While swimming, players can go through one block gaps similar to when using an elytra.
Item and experience
Items and experience orbs now float to the surface of water rather than sink, this makes dying underwater easier to deal with because all of the player's items now wait at the surface. This also allows for easy vertical item transportation.
Bubble columns
Magma and soul sand now make bubble columns when placed underwater. Magma naturally generates in underwater ruins and where underwater ravines meet lava, while soul sand does not currently generate naturally in oceans. Players are able to breathe while in bubble columns.
Magma bubble columns pull things down to the bottom; this includes items, players, mobs, and boats. Soul sand bubble columns rapidly push all entities, including players, to the surface.
Breath
The breath meter no longer instantly refills when entering air from underwater; this means it is no longer possible to instantly refill the air meter with a torch while underwater.
Vision
Water has been given unique colors depending on the biome they are in. Water is also now clearer, and the longer the player is underwater the clearer it becomes.
