Tool

He could lie below the surface for as long as twenty minutes, slowing his heart rate, slowing his bulk so that he needed almost nothing at all.

Without exertion, he might even be able to lie there longer, but twenty minutes, he knew for certain—much as he knew that he could run for five miles without rest amongst the high passes of Tibet, or for three days without pause across the blistering sands of North Africa’s Sahara.

Page 15 Chapter 2, Drowned Cities

I think the long immersion in water is helping him. It's amazing how long he's staying down without oxygen."

"They're designed for amphibious assaults," gthe general said. "They can stay down for a good twenty minutes like that. Maybe longer."

Pg 45, Tool of War, Chapter 5