Just over 1,000 miles from the coast of Australia lies New Caledonia, an island archipelago where the waters teem with life. This French territory, in the heart of the Coral Sea, is home to over 9,300 marine species, including dugongs, manta rays and venomous sea snakes.

Among them is the greater sea snake, which can reach nearly five feet in length and is more than capable of killing a human with a single bite. But such a fearsome capability doesn’t bother Monique Zannier, 75, one of a group of seven women, ages 60 to 75, who snorkel regularly in Baie des Citrons, a bay in New Caledonia’s capital, Noumea.

“The Baie des Citrons is our playground,” she said. “We are in it almost daily and we know all its nooks.”

What started as good regular exercise for Ms. Zannier has turned into a bounty of data and information for scientists studying the aquatic snakes. Researchers seeking new insights into the ecology of these marine reptiles have come to rely on the women, nicknamed the “fantastic grandmothers,” to help keep track of the hundreds of greater sea snakes that visit Noumea’s shallow-water bays.