First off, I like the concept. As far as I know there's no mechanical benefit to leaving one hand empty while using a sword. I love flavorful challenges but I just wanted to make sure you're aware.
If you're going to be staying out of the way, you might consider Sneaking and the Guerrilla talent. It's a great way to keep squishy characters out of the way as well as boost their damage. Still, her damage is going to be mediocre compared to a more focused mage or warrior.
That said, I suspect she's going to be a lot tankier than you expect. The Air spell can make up for a lack of armor and by the end even Glass Cannon mages become quite hard to kill. A Lone Wolf, non Glass Cannon character with a pile of HP is going to be pretty absurdly tough even without a shield and armor. You might even find yourself on the front lines eventually.
For spells, heals are okay but shields are even better - not least because you can cast them preemptively instead of waiting for someone to get hurt. On top of their stated benefits (resistances and such) they provide a health buffer that works against all damage. You'll see it as a colored bar surrounding your normal health bar. You get access to Earth Shield at level 7 and the others at level 10. Geomancy also has some fantastic cleric-y buffs like Bless, so definitely branch out from Water and take a look at the other schools.
