Richard Ellis, a polymath of marine life whose paintings, books and museum installations — especially the life-size blue whale at the American Museum of Natural History in New York — revealed the beauty and wonders of the ocean, died on May 21 in Norwood, N.J. He was 86.

His daughter, Elizabeth Ellis, said the cause of his death, at an assisted living facility, was cardiac arrest.

Mr. Ellis had no formal training in marine biology, conservation, painting or writing. But in fusing his artistic flair with an encyclopedic knowledge of ocean creatures, he became an invaluable, sui generis figure to conservationists, educators and those curious about sea life.

“Richard was an enthusiast, and he absolutely adored the natural world, especially the sea,” said Ellen V. Futter, the former president of the natural history museum, where Mr. Ellis was a research associate for many years. “He wanted everybody to share his appreciation and joy from the beauty of it, but also to feel the same sense of responsibility to protect it.”

Mr. Ellis spent much of his life traveling to exotic locales, where he bobbed around on boats and went diving in search of giant squid, great white sharks and other fantastical, elusive deep-sea creatures.

“If people understood the life, the importance, the habits of these creatures — whether sharks or whales or manatees — they would acquire a reverence,” Mr. Ellis told The New York Times in 2012. “I do it so people will say, ‘Wow, I didn’t know that’ or ‘Isn’t that cool! Look at what octopuses can do!’”

His photorealistic paintings of whales were sold in an art gallery and published in Audubon and National Wildlife magazines and in the Encyclopaedia Britannica. His more than a dozen books about marine life — especially his tomes on whales, sharks and tuna — made him, in the view of the best-selling author Simon Winchester, the “poet laureate of the marine world.”

Throughout his life, Mr. Ellis was never far from a major body of water. Growing up on the Rockaway Peninsula in Queens, he swam nearly every day in the Atlantic Ocean, weather permitting. The blue water, and what lurked below, frequently washed up in his daydreams.

“I would be sitting in class, learning about the Revolutionary War — except I was drawing swordfish,” he told a weekly newspaper on Long Island in 2015. “I didn’t think this was going to mark the beginning of my career, but it did.”

In 1969, the American Museum of Natural History hired Mr. Ellis as an exhibition designer and assigned him to help build a life-size blue whale to hang from the ceiling in the Hall of Ocean Life.

“I thought, ‘OK, how hard can it be?’” Mr. Ellis told The Times. “There must be all kinds of pictures.”

There were not. Mr. Ellis had to rely on drawings and photos of dead animals, an experience that convinced him that the only way to accurately depict oceanic wonders was to swim among them — even if they harbored a desire to eat him.

In the 1980s, wearing scuba gear and protected by a steel cage, he was one of the first ocean explorers to swim with great white sharks. He told The Times that he recalled “breathing extraordinarily fast because I’m sure the sharks are going to break through the cage and kill me.”

After that fear subsided, Mr. Ellis became filled with wonder.

“You don’t belong here, but it does,” he said. “And then you understand how beautiful it really is, and you spend the rest of your time staring at this animal or photographing it and thinking to yourself, ‘I am very privileged to be able to see this creature in its natural habitat.’”

Richard Ellis was born on April 2, 1938, in the Belle Harbor neighborhood of Queens. His father, Robert, was a lawyer and also worked at the United Transformer Corporation. His mother, Sylvia (Levy) Ellis, was also a lawyer but did not practice.

He spent most of his childhood swimming in the ocean.

“I had always been fascinated by the ocean and what lived in it,” he told The Times. “But most of the time, what lived in it was me.”

After graduating from the University of Pennsylvania in 1959 with a degree in American civilization, he joined the Army. He was stationed in Honolulu and, in his off hours, surfed and swam in the Pacific Ocean.

Mr. Ellis maintained an affiliation with the American Museum of Natural History for much of his life, but he made his living painting, writing and illustrating books. His output was prodigious.

“The Book of Whales” (1980) tells the complex history of nearly every species of whale, accompanied by his illustrations.

In “Monsters of the Sea” (1994), the author and naturalist Janet Lembke wrote in a review in The Times, Mr. Ellis aimed his “insatiable curiosity” at “legend-hallowed behemoths: the leviathan, the polyp, the man-eating elasmobranch (otherwise known as the shark), all manner of sea serpents (including Nessie, the Loch Ness monster) and some great, stranded lumps of flesh called ‘blobs’ and ‘globsters’ for want of more precise names.”

“Tuna: A Love Story” (2008) tells the story of how a fish capable of swimming 55 miles per hour became an overfished commodity.

“To biologists,” Mr. Ellis wrote, “the tuna is the epitome of hydrodynamic excellence; it is fast, powerful, streamlined, and equipped with specializations that enable it to perform its duties better than any other fish in the ocean.”

To humans, it is tuna salad and sushi.

“What I do is I paint the things I admire,” Mr. Ellis said on the NPR program “Talk of the Nation” in 2008. “Other people shoot them, some people fish for them. I paint them.”

Mr. Ellis married Anna Kneeland in 1963. They divorced in 1981.

In addition to his daughter, he is survived by Stephanie W. Guest, his companion since 1989; a son, Timo; Ms. Guest’s children, Victoria, Vanessa, Fred and Andrew Guest; six grandchildren; and his brother, David. He lived on the West Side of Manhattan for many years.

Mr. Ellis appeared on the CNN program “Larry King Live” in 2001 after a shark bit an 8-year-old boy in Pensacola Beach, Fla.

“They are dangerous when they hungry, right?” Mr. King asked him.

Not exactly.

“They’ve been around for roughly 300 million years,” Mr. Ellis said. “And if something moved in the water, it was edible to a shark.”

It’s not the fault of sharks that people started swimming in their water.

“If it moves in the water and you’re a shark,” Mr. Ellis said, “you can eat it.”