Promontory Point

Alright, there's a bit of confusion finding this place.

If you go straight to the question mark near the sea floor, there will be a large rock formation. You'll swim all around it and not find the turn in. WRONG ROCK. Swim alllll the way up yonder. The turn in is near-ish to the surface, not close to the ground at the original rock formation. You'll be facing the giant turtle creature L'ghorek.

42, 37

I would emphasize that the quest turn-in NPC is standing on the surface of the rock formation, and not the surface of the water.

Guide to Vashj'ir Abyssal Depths Alliance quests:

The entire Vashj'ir line starts in Stormwind with Call of Duty.
The immediate pre-requisite to this chain is Full Circle from
Shimmering Expanse.

At any time, kill a Hellscream Seadog to get Claim Korthun's End.

Main chain:
Bio-fuel
Fuel-ology 101 ( use 2 remora and 3 hammerhead oil)
The Brothers Digsong
Phosphora Hunting
A Lure and Coldlights Out
Optional: Enormous Eel Egg should have dropped, which starts
a quest leading to The Brothers Digsong 2: Eel-Egg-Trick Boogaloo

Next: One Last Favor which also unlocks optional quests

• 
  
• Sira'kess Slaying
• 
  
• A Standard Day for Azrajar
• 
  
• Treasure Reclamation
• 
  

Those Aren't Masks
Put it On
Several optional quests:

• 
  
• Promontory Point
• 
  
• The Wavespeaker and Free Wil'hai
• 
  
• Clearing the Defiled
• 
  

Scalding Shrooms and Into the Totem
... It Will Come (also unlocks Fiends from the Netherworld)
Unplug L'ghorek
Communing with the Ancient
Runestones of Binding and Ascend No More!
Optional: Twilight Cage Key drops to start Prisoners
Twilight Extermination
All that Rises
Back to Darkbreak Cove
Defending the Rift (gives 20,000 Leagues under the Sea and ends the Vashj'ir quests)