[2] He was born in Barrie, Ontario, but grew up in Toronto, where his family moved after his father, a Royal Canadian Air Force instructor, died in a plane crash when MacInnis was a few months old.
[1][5][8] He spent his internship at the Toronto General Hospital,[1][6] where an experience with a tunnel construction worker suffering from decompression sickness helped to point MacInnis toward his post-graduate studies in diving medicine.
MacInnis arranged for the worker, John McGean, to be transported to a pressure chamber in Buffalo, New York, where he was successfully treated.
[9] After his junior internship, MacInnis hoped to work with inventor and entrepreneur Edwin Link in the field of deep diving, but had no idea how to reach him.
[6][8] MacInnis received a Link Foundation Fellowship to study diving medicine under Christian J. Lambertsen at the University of Pennsylvania.
Sténuit and Lindbergh stayed in Link's SPID habitat (Submersible, Portable, Inflatable Dwelling) for 49 hours underwater at a depth of 432 feet, breathing a helium-oxygen mixture.
[6][10][11][12] The dive was successful, although MacInnis made a potentially grave error by placing a cover without a pressure-equalizing valve on a carbon dioxide-filtering device.
[6][10][13] In September 1967 MacInnis took part in a classified Ocean Systems mission aboard Deep Diver on the Grand Banks south of Newfoundland.
The mission was called off due to rising winds, and Deep Diver was barely brought safely back aboard the Canadian Coast Guard vessel CCGS John Cabot.
[6][10] In 1968 MacInnis took part in a saturation dive aboard the Hydrolab underwater habitat with two other aquanauts, spending 50 hours at a depth of 50 feet,[14] and in the search for the lost submarine USS Scorpion.
[2][6] In 1969 MacInnis served as a medical consultant on the U.S. Navy's SEALAB III project,[1][8] and was on site the day after the death of aquanaut Berry L.
[clarification needed][7] MacInnis designed and built Sublimnos, the first Canadian underwater habitat,[16] which was placed in Georgian Bay near Tobermory, Ontario, in June 1969.
In July 1969 MacInnis attended the Apollo 11 launch at Cape Kennedy, then traveled to Tobermory, where he dived to Sublimnos and looked up through the water at the Moon at the very moment the astronauts were walking on it.
[20] In March 1971 MacInnis was a member of a Canadian Broadcasting Corporation crew filming Harp seals in the Gulf of St. Lawrence.
[6] Also in 1971, MacInnis helped oversee the successful decompression of Ocean Systems diver Bill Maltman after he was fouled in wreckage while taking part in salvage operations in the wake of the crash of a B-52 into Lake Michigan.
[22] MacInnis visited the Soviet Union for the first time in autumn 1973 as part of a scientific exchange program, and showed a short film about his underwater polar research in Moscow and Leningrad.
[2] While diving in 1975, MacInnis found a fragment of the Breadalbane, the northernmost known shipwreck in the world, a British merchant ship that sank in the Arctic in 1853.
[31][34] Canadian engineer Phil Nuytten's atmospheric diving suit, the Newtsuit, was used to retrieve the bell from the ship, replace it with a replica, and put a beer can in the Fitzgerald's pilothouse.
[36] In 2003, MacInnis accompanied filmmaker James Cameron on the Disney-IMAX expedition to the Atlantic and Pacific Oceans which resulted in the 3-D film, Aliens of the Deep.
[7] In 2005, MacInnis joined Cameron's Discovery Channel expedition which explored the last unseen rooms inside Titanic and broadcast live television pictures from the wreck.
[8] In March 2012, MacInnis served as expedition physician for Cameron's solo dive to the bottom of the Mariana Trench in the Deepsea Challenger submersible.