So, breathing underwater is basically Doesn't Breathe (Gills) plus we need to add in mana sensitive. Very simple.
Breathe Water Buff (-60%): Doesn't Breathe (Mana Sensitive, -10%; Gills, -50%) 
This will be a buff that lasts for three minutes at least. So we need to afflict someone with this advantage for three minutes and the affliction needs to have the Sorcery modifier.
Breathe Water (+80%): Affliction(Sorcery -15%; Advantage, Breathe Water Buff +80%; Increased 1/2D, 10x, +15%; Fixed Duration, 3 Minutes, +0%) 
So this spell costs 1 energy and requires a roll against an innate attack skill to work. It lasts for 3 minutes.