Sam Fisher- HUD and Vest

  He picked out a few landmarks he'd chosen from his map before the mission, confirmed their position on his HUD display, then flipped over and dove.

  TEN minutes later, he stopped swimming and coasted to a stop. He adjusted the compensator on his vest until he was neutrally buoyant, hovering motionless in the water. Aside from the glow of his HUD, he was surrounded by absolute blackness. Night diving could be an exercise in mind control, Fisher knew. Without any external reference points, a vertigo-like confusion can quickly take over. Fisher had seen the bravest of men, divers with hundreds of hours of bottom time, panic while simply floating motionless in dark water. Even he could feel it nipping at the edges of his mind: a primal urge to rush to the surface. He quashed the feeling and focused on his face mask; the soft green glow was reassuring. If his navigation was accurate, the sea fence lay directly ahead.

- Checkmate, Chapter 6