Multiple bugs:
1.Get an Elytra
2.get a trident
3.World rain
4.Enchant the trident with Riptide
5.Equip elytra
6.Fly High and to the sides or to the sides at max speed.
7.Game stops generating chunks
8. you can fly under 0 high, then cant use the trident anymore
9.Game crashes or doesen't
10.try any commands none working
11.Try to save and exit = Game crash

Water textures:
Watch water from below, It is gray

Very big differences in performance from 1.12.2 to 1.13
Chunks load very slowly
Fps are lower
Any non default world generation has an 80%~ of probability to crash (lets you view the loading screen forever, not tried more than 20 minutes on background)

This might be the reason I can exit the chunk loading when flying

I would really recommend you implementing optifine code to the game without any need for moding

Flying under water makes you swim at super speed under water making it annoying to build
Also didn't think that building under water on survival will be also annoying if your camera goes up and down: Add a swim trigger button and maybe make it possible to crow on ground with it.

Different under water chests contain a map of the same treasure

There are still blocks that allow to air pockets form such as doors anvils...

Zombies stop chasing you under water they wait until they convert to drowned and then continue

Trident picked up that was thrown by a drowned has full durability

Drowned that has a trident can keep throwing them infinitely

Drowned needs an animation when swimming

Phantoms tried to bite me on creative

Sometimes a bubble n11 might appear when getting air

Not a bug? trident not working out of the water... unless it's raining Why?

Rain does not emit sound on water

(Old bug) lightning sometimes emits a sound that cuts too early, breaking ambient. Also add code to don't allow lighting volts to spawn in the view area of a player in too close intervals. Sometimes lightnings make sound and on a server you can hear it everywhere.

New lighting bug? Under water at the Icebergs biome lightning bugged the light levels of an area, making it too bright.

Sometimes there is light but no source block or a source block that doesn't emit light (this happens with world edit or caves [lava])

fish Should not despawn, I've seen in an ocean a bunch of them to despawn and 3 seconds later another bunch spawn

there are still drowned floating arround at day time