A mob riding a squid wont follow the squid if it dives, it stays floating on top.
map demonstrates this
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Unless this gets fixed speciffically, i doubt it will get fixed, because the developers dont change every single game mechanic when they make updates, and is has been a bug for several updates.
I am not on a computer i can use to check if this is still a bug, but it probably is.
Is this still a concern in the current Minecraft version 14w21b / Launcher version 1.4.4 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Is this still a concern in the current Minecraft version 1.8.1 Prerelease 3 / Launcher version 1.5.3 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
THIS IS STILL AN ISSUE

This has been marked as a duplicate of MC-72440 due to visibility and activity on the respective issues.
still not fixed
Is this still a concern in the latest Minecraft version 13w48b? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.