Reproduce:
Summon a squid
Run command
/attribute @n[type=squid] minecraft:movement_speed base set 0Squid can still swim
Related issues
relates to
MC-247426
Turtles can still move despite having their movement speed attribute set to zero
MC-2112
Adjusted speed does not affect sprint-jumping/falling/floating with levitation/floating with NoGravity
MC-40949
The movement_speed attribute has no effect on flying mobs
MC-43269
The slowness and speed effects don't affect movement in water
MC-269477
Movement speed attribute does not affect swim speed and horizontal fall speed
Comments
Dhranios
NeunEinser
@unknown It does not appear to be swimming in general, as fish do respect this attribute as an example.
[Mojang] Triage Team
Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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Swimming speed of entities is not controlled by (that) attribute(s), just like moving while in the air. That attribute only controls walking speed. (The attribute is actually misnamed.)