Africa's Nile crocodiles don't grow to more than a 1,000 pounds by being discerning about what they eat. So if you're going to scuba dive with these primordial reptiles, there are a few important things to know: First, crocodiles have terrible eyesight, which means they typically hunt at the surface, where they can spot prey in silhouette against a bright background. Second, they become sluggish and reclusive in cold water, which means the safest time to see them in Botswana's Okavango Delta, where they number in the thousands, is in July and August, when the water is high, cold, and clear.

All that said, it's not easy – or free of risk – to swim with these ancient beasts.

"They're very hard to see because they look like pieces of wood," says Amos Nachoum, a Brillo-haired Israeli wildlife photographer and president of Big Animals Expeditions, a company that takes groups to see predators like sharks, leopard seals, and snow leopards without cages or protective gear. Still, once spotted, they don't reflexively hide. "We can almost get as close as touching them," he says.

An obvious question might be: Why would anyone voluntarily pursue such an encounter? While it may not take a lot of skill – divers must be moderately fit, able to dive in strong currents, and have an ample supply of nerve – diving with crocodiles is, undoubtedly, a singular experience. Nachoum, who's logged hours in the water with crocs, has a quick answer.

"From the moment I decide to roll back into the water, the sense of fascination and adrenaline is taking over," he says. "The excitement and the rush pushes me closer toward the croc until I am so close the camera cannot focus anymore – only 10 inches face to face. That is transcendence."

More Information: Big Animals Expeditions's next trip to Botswana to dive with crocodiles takes place June 28–July 7, 2014 and costs $14,900. The company also leads trips to dive with leopard seals in Antarctica, to dive with great white sharks in Mexico, and to hike to see polar bears in Canada.