Java Edition guides/Update Aquatic

This guide is a simplified overview of all the changes in 1.13, which assumes at least some familiarity with 1.12.

New
1.13 adds several new mobs to the game, here is a list of the mobs to look out for:

Changed
Nearly all the biomes were renamed, here is a list of the new biome names versus the old names, note that the debug screen will no longer show the biome name but will rather show the internal biome id:

For more information see 1.13/Flattening.

Water
The physics for water has 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, this 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 since all of the player's items will be waiting at the surface. This also allows for easy vertical item transportation.

Bubble Columns
For more detailed information see Bubble Columns.

Magma and soul sand now make bubble columns when placed underwater. Magma will naturally generate in underwater ruins and where underwater ravines meet lava, while soul sand does not currently generate naturally in oceans. Players are able to breath while in bubble columns.

Magma bubble columns will pull things down to the bottom, this includes items, players, mobs, and boats. Soul sand bubble columns will push all entities, including players, to the surface at a very fast rate.

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.