Expected:
As an arial conjurer I cast projection. In astral form I move below the surface of the water and into an air bubble then back into water once again. Nothing of the material plane affects me.
I am not affected by doors, heat, frigid cold, disease, and I should not be affected by water.

Actual:
I cast projection. I move underwater then into an air bubble. Hydrophobia now affects me. I am trapped by the material world. The ticker is counting down. I may die if the projection spell wears off.

Honestly, phobias are a mental condition, and have nothing to do with reality or rationality.

Projected or not, going underwater scares the crap out of you.

Just as a person with true arachnophobia cannot bring themselves to pickup even the most teeniest of harmless spiders, you can't bring yourself to go underwater, even though it's perfectly harmless and even though it may mean your death if you don't!