Directly up, there's Sonja's hand, grasping Balthier by the back of the shirt and tugging him toward the surface. She's capable of holding her breath for much longer than most people, but it's harder to use those rites here in the dead zone, so she's relying primarily on experience. That's fine with her, or so she tells herself--nothing she can do about it right now except survive, and drag Balthier with her.

Close to the surface, another large eel approaches, attracted by the scent of blood. Sonja releases Balthier in order to deal with it, and ends up catching the creature between her knees in order to keep it from snaking up and attacking her--or escaping. She has the desire to try to kill all of them, even if any of them want to get away. The sword is overkill when a knife would do, but Sonja is always overkill and furthermore it's close on hand, so she brings it directly through the eel's eyes, suspended higher in the water by its thrashing weight.

Its blood sprays up toward the surface. In its dying throes, it does manage to gash her stomach and knock her against the underside of the boat, but she shoves away and abandons the dying eel to find Balthier again.