Sports photographers who shoot underwater used to free-dive to make these adjustments; now many, including Bello, Rose and Pretty, are certified scuba divers. That has given them the comfort of extended time underwater to perfect the art of capturing the world’s best swimmers from below. But it also leads to an odd sight every night at the London Aquatics Centre: a glass-and-plastic reef composed of 10 cameras and a platoon of frogmen who enter the pool soon after the last race to tend to them.