The mobs can't swim in the water
Linked issues
is duplicated by
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Can confirm with spiders. Also relates to MC-47824
Confirmed for 14w10c
Confirmed for 14w11b. Mobs are significantly slowed down by water.
Duplicate of MC-48616.

Can't swim != Swimming too slow
The dogs jumped in the water but almost couldn't move. They also never ported over to me as I moved away from them. I went 1000 blocks before I noticed they weren't following me. When I went back they were still stuck in the water where I first got on my boat.
(MC-48616)

Yes, but this ticket (and it's duplicates) is about mobs sinking to the ground and drown.
Confirmed for 14w20b with blazes in lava
It's only the blazes in water and lava. Every other mob is fine, besides silverfish and other smaller mobs.
Endermites can't swim either in 14w21b. They sink at the same speed as items (I'm unsure if this is intended or not).
Confirmed in 14w27b
Also confirmed for 14w27*, this effects mob farm behavior and should be fixed promptly.
confirmed for 14w28b
confirmed for 14w29a; why is nobody voting for this?
confirmed for 14w29b
confirmed for 14w31a
mobs can't swim AND can't jump on a 1 block higher level
the baby zombies can move in water but very slowly
it seem to be that the movement speed of mobs is decrease in water.
because baby zombies are faster, the can swim slowly and other mobs can't move because they are to slow
I'm seeing this as well on 14w32a. If I hold wheat, cows look at me as if they want to follow, but they don't move, or move very slowly. I basically have to push them across water.
Squids no longer affected by water currents in farms like this: https://www.youtube.com/watch?v=yducYP9Whmc
Can someone clarify what the issue is and how to reproduce this? Because mobs are floating (swimming) for me. Including Silverfish (they float and don't just sink to the bottom and drown - even in downward flowing water).
Also, this is NOT about them swimming too slow, as that's MC-48616.
This bug is fixed in the prerelease. All mobs can swim. They just swim slowly, that's another issue.

The issue "Mobs can't swim" is in fact fixed.
"Mobs swimming too slow" is MC-48616.

"Blaze not swimming in lava" separated from this ticket: MC-48907
Reopened.