First you need to get to a place like this, one side with water and one side with nothing. This is north of darnassus.
When you are on the left side with nothing, you regen fatigue.
So the point is when you have low fatigue, go to the nothing-part and pop slowfall (not necessary) and when your fatigue is full you can simply blink and change direction to enter the water part again. Since you used slowfall your height haven't decreased much and you should be able to reach the surface with over 50% fatigue remaining.
Managems and evocation should be about infinite mana, you dont regen hp/mana while losing fatigue.