The mobs can't swim in the water
Linked issues
is duplicated by 10
relates to 2
Comments 25

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.
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.