Laria gives you 3 Millrush Brews to make this quest do-able, because there are NO air vents inside this cave! Each Brew will give you 8 minutes underwater and this quest should not need more than one. Sadly, any left over are removed from your inventory when you complete the quest.
Getting in and out of the cave is not so difficult, but if you are unlucky you will run into the wandering Elite in the pond, the Blacksap Ancient. If this happens just accept it, bend over gracefully, and take it like a man.
Directly over the cave entrance you will encounter Ol' Ness (level 25).
There are 4 or 5 level 24 Grimeskins in the tunnel and you will have to clear them as you go. The final mob at the end of the cave, Rift Mob:, is level 25.
At the end you will finally reach Athana's Chest and recover Athana's Shard.