Well, most of you probably watched the "Blink Fixes" video which Nostalrius released some time ago. I did too.
There is a part where they detail "3D Blinking underwater" - basically, you get to blink in the direction you point while being underwater. Let's watch it in their video together:
https://www.youtube.com/watch?v=Ny2Hogb_k98#t=77
Now let's direct ourselves to a post made on the official wow forums in 2011:
http://us.battle.net/wow/en/forum/topic/2674977856
Interesting. Let me quote some passages from it:
The problem with this ability currently is that it doesn't register a 3-dimensional plane. What I mean by this is that while underwater or during a phase in combat when you are able to move in 3-dimensions such as Kael'thas in Tempest Keep or Al'Akir in the final phase you find yourself unable to blink in the direction that you would like to.
If you decide to blink while in these environments the game puts you on a horizontal plane from the bottom of your character and only allows you to move horizontally from that position.
It honestly took me in the range of 10-15 minutes to find this post online, and I'm sure there is way more information out there - heck, from looking at that post, this functionality probably doesn't even exist in Warlords of Draenor and never has!
TLDR; If you make custom changes to Blink, don't call it a 1.12.1 server.