Our captain watched with some consternation as an unidentified vessel, gray with no markings, headed straight toward our vessel, anchored more than 50 miles off Cuba's southern coast. Others in the crew speculated nervously about the approaching boat, never previously seen in these parts. The boat pulled alongside and two imposing figures boarded, both in olive military uniforms. A mustachioed representative of the Ministry of Interior stood beside his taller colleague whose uniform, like the boat that carried him, bore no markings at all. A sidearm hung imposingly from his belt. He turned to the captain and requested to meet with Robert F. Kennedy, Jr.

Robert F. Kennedy, Jr. makes a journal entry aboard ship during The Explorers Club's expedition to document unexplored waters off southern Cuba in 2014.David E. Guggenheim

At that moment, Kennedy—a leading environmental activist, president of Waterkeeper Alliance and son of the late Senator Robert F. Kennedy—was 90 feet below the surface with the rest of our group, observing a dozen or so Caribbean reef sharks tracing mesmerizing circles about us. We were carrying the flag of The Explorers Club, visiting and documenting previously unexplored coral reef ecosystems in Cuba's southern waters.

After returning to the boat, the mission of our mysterious guests was revealed. We had been visited by a representative of former Cuban president Fidel Castro's personal guard who had a letter from the Comandante for Kennedy. Mission complete, they posed for a quick photo and departed on the 50-mile journey back to shore and the six-hour drive back to Havana. They had traveled an incredible distance to find us and hand-deliver a letter. We were obviously quite curious as to its contents.

Our captain, Arjel, and the two soldiers that delivered the letter from El Comandante to our vessel.David E. Guggenheim

A few days earlier, Kennedy, Jr. and his family had visited with Castro, who welcomed them warmly. Nearly 52 years prior, Robert Kennedy, serving as U.S. Attorney General, and his brother, President John F. Kennedy, were within a whisker of war with Cuba and the Soviet Union during the Cuban Missile Crisis. The quiet Castro-Kennedy, Jr. meeting was historical. Relations between Cuba and the U.S. were warming, though the dramatic announcement of normalization of diplomatic relations would not occur for another six months.

Kennedy, Jr. shared the letter with me, a polite set of Castro's reflections on the meeting and kind words for Kennedy and his family. What I found especially significant in the letter was his discussion about oceans:

"For many years I was a passionate spearfisherman without the proper awareness of the beauty and value of coral reefs. Through this I knew some of the experiences of Jacques-Yves Cousteau, who in such a way fell in love with the sea that ended up becoming one of the most famous defenders of the life and the value of the seas. Today it is known that the sea is one of the largest and varied sources of protein foods. These factors helped me understand the importance of the services you have rendered to the people of the United States and other nations of the world in their struggle to protect the environment."

The influence of Cousteau on Castro has been a recurring theme I have heard from Cuban colleagues during my many years working in Cuba. Castro read and was influenced by Cousteau's books and, in 1985 when Cousteau visited the island to make a documentary, the two finally met and shared a special friendship. Castro granted Cousteau with rare privilege during his visits. Cousteau and his team became the first non-Cubans to pass through the gate of the U.S. Navy's Guantanamo Bay installation since 1962. He is reported to have freed dozens of political prisoners at Cousteau's request. And Castro spent a great deal of time with Cousteau, dining with him aboard his vessel, Calypso.

Jacques-Yves Cousteau's Vessel, Calypso, in Havana Bay (1985)Cousteau Society

In the late nineties, aboard another research vessel visiting from the U.S., Castro reflected on his friendship with Cousteau and said, "You know, he loved exploring Cuban waters because of our protection." In the Cousteau documentary, Cuba: Waters of Destiny, Cousteau is clearly taken with what he observes in Cuba: "My first dive in the waters of Cuba serves as a moment of truth…around me, large fish among flourishing coral, a reef more rich than any I have seen in years," a stunning reminder that even 30 years ago, the unraveling of coral reef ecosystems in the Caribbean was well underway. Today it is estimated that the Caribbean has lost half of its coral cover. Spared in part by a history that has caused Cuba to develop profoundly differently than the rest of the Caribbean, coupled with world-class environmental laws, many of Cuba's coral reef ecosystems have been spared the demise observed throughout the Caribbean.

Before allowing the Calypso to depart Cuba's waters, Castro challenged Cousteau, asking him why he didn't have a Cuban scientist aboard. Consequently, Cousteau later welcomed Dr. Gaspar Gonzalez Sansón, former vice director of the University of Havana's Center for Marine Research, to serve as a visiting scientist aboard Calypso in New Zealand. Years later, Dr. Gonzalez would become our co-principal investigator for a decade of expeditions off Cuba's northwestern coast and regaled us with hilarious tales of a Cuban among Frenchmen aboard Calypso.

Captain Jacques-Yves Cousteau (left) and Dr. Gaspar Gonzalez Sansón (right) on the bow of the Calypso in New Zealand. At the request of Fidel Castro, Dr. Gonzalez served as a visiting scientist during Cousteau's "Rediscovery of the World" expedition.

The friendship of Cousteau and Castro continued and strengthened in environmental solidarity at the Rio Earth Summit in 1992 where Castro delivered a sharply-worded and uncharacteristically brief address, imploring the developed world to "stop transferring to the Third World lifestyles and consumer habits that ruin the environment. Make human life more rational." In early 1998, less than six months after Cousteau passed away, Castro fondly remembers a playful encounter with Cousteau at the Rio Earth Summit: "They have all the heads of state lined up for a 'photo op' in Rio, and I pulled him [Cousteau] up with me, and say, 'Captain, join this picture in the 'photo op' because most people here know nothing about the environment. And he came up and was in the 'photo op' with all of us."

President Fidel Castro and Captain Jacques-Yves Cousteau in a playful exchange at the Rio Earth Summit in 1992.

In July 1997, Cuba enacted Law 81, the Law of the Environment, a truly impressive set of laws and regulations meant to reverse the environmental damage from prior decades and chart a path of sustainability. Within a decade, Cuba banned the destructive fishing practice of bottom trawling from its waters. Today, Cuba has nearly met its goal of protecting 25 percent of its marine waters in marine protected areas, one of the largest percentages in the world. (In comparison, the world average is currently 2-3 percent). Many Cubans attribute Law 81 and Cuba's ongoing commitment to the environment to Castro's environmental ethic, which the Comandante, in part, attributes to Cousteau.

With the passing of Castro and a possible retreat on Cuba relations by an incoming Trump Administration, there is a growing uneasiness about Cuba's uncertain future. Facing profound economic need and unprecedented growth pressure, especially in response to plans more than triple tourism by 2030, Cuba will be put to the test in the months and years ahead. For now, Cuba remains a green, unspoiled jewel in the Caribbean. It is a place where policy is still informed by science and fact, and decisions governed by its laws.

By 2014, it had been some time since Castro had last donned a mask and personally explored Cuba's waters, but it was clear that his passion and curiosity for the sea was as strong as ever. In his letter, Castro made a simple but urgent request of Kennedy, Jr: "Today, I beg you, if you have a few minutes, tell me about the general impression of what you have seen on the bottom..." Several weeks later, Kennedy complied and assured the Comandante that for now, Cuba's marine ecosystems were still healthy and spectacular.