The French firm Air Liquide held the patent on the original "Aqualung" until the patent expired some time around 1960-63. The term "aqualung" as far as is known first appeared in print on page 3 of Jacques-Yves Cousteau's first book, The Silent World (1953). Public interest in scuba diving began around 1953, in response to a National Geographical Society Magazine article in English-speaking counties, and in France a movie, and perhaps The Silent World. Like with some other registered trademarks, the term "aqualung" became a genericized trademark in English-speaking countries. Presumably lawyers for Cousteau or Air Liquide could have slowed or stopped this genericization by taking prompt action, but this seems not to have been done in Britain, where Siebe Gorman had the British rights to the trade name and the patent.
In the United States the term Aqualung was popularized by the popular television series Sea Hunt, which never said that an aqualung could be called anything else or could be made by anyone else but the company that supplied the fearless Mike Nelson. The word "aqualung" was commonly used in speech and in publications (including the British Sub-Aqua Club's official diving manual) as a term for divers' open-circuit demand-valve-controlled breathing apparatus (even after Air Liquide's patent expired and other manufacturers started making identical equipment), and occasionally also for rebreathers, and in figurative uses such as "the water spider's aqualung of air bubbles". The word got into the Russian language as the generic noun акваланг ("akvalang"). In the United States, U.S. Divers managed to keep "Aqualung" as a trademark, and the acronym "SCUBA" ("Self Contained Underwater Breathing Apparatus", originating in the United States Navy, where it meant a frogman's rebreather) became the generic term for that type of open-circuit breathing set, and soon the acronym SCUBA became a noun — "scuba" — all in lower-case. "Scuba" was a trademark for a time, used by Healthways, now known as Scubapro, one of U.S. Divers' competitors.
Presumably, anyone who uses "aqualung" generically now can expect a polite but firm "cease and desist" letter from a law firm representing U.S. Divers.