John Randall had wandered off again. He had been diving with family and friends in Malaysia in 1998 when he disappeared. After a long search, the group spotted him walking back over a sandbar. One of the dive masters said in exasperation, “Who is this old guy?”

Hank Bauman, Randall’s former son-in-law, had a ready answer. “Well, you see all those fish books in your shop?” he said. “And under the name of the fish, you see the name ‘Randall’ on a lot of them? Well, that ‘old guy’ is Randall.”

Randall, who swam the waters of the world to add new creatures to science’s tally of ocean life, died April 26 at his home in Kaneohe at 95. His daughter, Loreen Randall O’Hara, said the death certificate cited “undetermined natural cause, probable lung cancer.”

Beginning his scientific career in the 1950s as the young field of scuba diving was opening the ocean depths to exploration, Randall went on to name 30 new genera and 834 new species of fish.

By comparison, the second most prolific ichthyologist “has a lifetime new species count of less than one-third of Jack’s,” Richard Pyle, senior curator of ichthyology at the Bishop Museum in Honolulu, wrote in an email.

And, he noted, Randall’s precision in recognizing tiny differences between specimens, and his encyclopedic knowledge of the field, meant that his species identifications were unusually durable; 97% of them are still regarded as valid, in contrast to the 50% to 60% recorded by other giants in the field. He was the author of 942 papers — “more publications than any other ichthyologist (of any subdiscipline) in history,” Pyle said.

Since finding new species is becoming less common today, he added, the record is likely to stand.

“Jack was the greatest ichthyologist in history. Period,” Pyle said. He created a memorial page for Randall that has become a virtual gathering place for hundreds of friends and admirers around the world.

Relentlessly focused on his work yet generous to others, Randall sparked the imaginations and launched the careers of generations of self-styled “fish nerds.” Among them was Randall Kosaki, a research coordinator for the National Oceanic and Atmospheric Administration, who approached him in the mid-1980s with what he hoped was an undiscovered angelfish.

As a “lowly graduate student” at the University of Hawaii, “I had my fingers crossed,” Kosaki said.

Randall agreed that it was a new species, and a delighted Kosaki assumed that Randall would write a paper on the find and perhaps share authorship. “Oh, no — you found this,” Randall said. “You take the credit for it.” Then he walked Kosaki through the process of writing his first published paper.

“Jack changed the course of my career,” said Kosaki, a deputy superintendent of NOAA’s Papahanaumoku­akea Marine National Monument. While he did not follow Randall into the field of taxonomy, he said, “He gave me the encouragement that young and insecure graduate students with an inferiority complex need.”

Randall was impressively athletic, showing a stamina in diving that left younger companions in exhausted wonderment. Pyle recalled that as a graduate student in his 20s, he and a fellow student struggled to keep pace with Randall, then nearly 70.

“I vividly remember feebly crawling onto the swim deck of the ship after Jack finally ended his night dive (our sixth dive of the day), only to see Jack switch out tanks for his second night dive of the evening,” Pyle wrote on the memorial page. “I barely made it to my bunk before collapsing, but Jack continued to process and photograph his treasured specimens into the wee hours of the morning.”

The next morning, Randall was back in the water before breakfast.

John Ernest Randall Jr. was born May 22, 1924, in Los Angeles, to John and Mildred (McKibben) Randall. His father was a contractor, his mother a homemaker.

“She took my father fishing, which began his fascination with fish,” O’Hara said.

After graduating from high school, he served in the Army from 1943 to 1946 and then attended the University of California, Los Angeles, earning a Bachelor of Arts degree in zoology in 1950. After college he sailed a 37-foot ketch he had refurbished from California to Hawaii with a small crew. In 1955 he earned a Ph.D. in marine zoology from the University of Hawaii at Manoa.

At the University of Hawaii, he met a fellow graduate assistant, Helen Au; after a lab session that involved dissecting chickens, he invited her back to his sailboat, where they had a chicken dinner. They married in 1951.

She survives him, as does O’Hara and a son, Rodney, as well as four grandchildren and two great-grandchildren.

After research fellowships and teaching appointments that took him to Yale University, the University of Miami and the University of Puerto Rico, Randall joined the Bishop Museum in 1967 as an ichthyologist. He became chairman of the museum’s zoology department in 1975 and remained senior ichthyologist until his retirement in 2009.

Sylvia Earle, an oceanographer and former chief scientist at NOAA, said in an interview, “It was a privilege to know him and see him where he was most at home: underwater. Even though I had to hurry to catch up.” His perception of fishes’ features was so refined, she added, that he could distinguish individual faces, the way that humans recognize one another.

His breadth of experience meant that “he was a witness to change,” she said. He saw firsthand the damaging effects of overfishing and global warming. “He could see the decline,” she said, “and shared with many of us the grief of seeing the ocean unravel.”

In his later years Randall found it difficult to walk but not to swim. Brad Tarr, his former curatorial assistant, recalled diving with him in 2000, which involved arduous preparation.

“He struggled into the ­water,” Tarr wrote on the memorial page, “then immediately transformed into a fish — he had no physical limitations once in the ocean.”