water acts exactly like air on a LAN server as a guest; players still lose oxygen and take damage when it runs out, but have to get above the surface via blocks.
pictures are of me falling, taken about 2 seconds apart.
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Did this only happen when on a LAN server?
I run a whitelist server, yes
Edit: and it happens from the host machine or from a remote client

Same here, very strange. There's no way to start swimming, as far as I can tell.

Same seed as a single player is fine, but on a multiplayer server swimming doesn't work at all.
verified; unable to swim if underwater. player sinks to bottom as if it was an air block. also will take fall damage if landing in water. sound effects and breathing act as if under water. boats sink like a rock once you board them. LAN/Server 18W19A
Another potentially related symptom of this issue is found when fishing. The bobber (float) does not remain on the surface, but immediately sinks to the floor of the body of water you are fishing in. The remaining fishing mechanics remain intact..i.e. the surface of the water still shows the approaching fish and you hear the subsequent 'strike' by the fish and right clicking to catch the target also still works.
Found a temporary workaround for guests. If they first enter a single player world and swim, then leave and enter the LAN multiplayer world they are able to swim like normal.
Thanks Cam P!
They better fix this bug because it is game breaking for servers.
also the fish animation is like they are not in water
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Lava also is blue and looks like water. Potentially related to this since it is multiplayer only and deactivated in the same way as this glitch.
If you 'fix' it by starting a single player world and then go back to multiplayer, is it fixed forever then or do you get the issue again after awhile?

It comes back again, Agnes. At least that was the way it was in 19b. You need to do it every time you launch your game.

What CreeperMagnet said. As long as you are still in that same session, you're fine – but you have to go into single player first every time you launch your game.
Ok, thanks!
I have issues reproducing this in 20a. But you all can reproduce the issue in 20a in the same way as in 19b?

Definitely still a problem in 20a, I tried it last night.

Ok I just tried it now, and it worked! The difference between last night and this morning is that last night, when I had to go into single player mode first, I was not the only person on the server. Today, I am. Hopefully that is some help!
Ok thank you Diane!
If anyone get this issue again, please let me know how you managed to reproduce it. Thanks!
18w20c I can swim, place and use boats. seems fixed (but new bugs have appeared in the water)

This bug doesn't seem to be affected by LAN or a server for me. Converting an 1.12 world to 18w20c also seems to work fine. This bug did affect worlds from older snapshots for me though!
Affects 18w21a on a world last opened in snapshots.
EDIT: Swimming in singleplayer first does not fix the problem as the reporter above claims.
Thomas, it worked in earlier snapshots, so thanks for telling me that it does not work now
Is present in 18w22b
Can confirm in 1.20.4 - Any fixes? it seems that when it breaks, it breaks across different instances of minecraft.
MC-129410 - Experienced the same. I also tested with boats. They float when placed but sink once you enter. they remain on the bottom until broken and then the drop will float.