I am going to reinforce what Dr. Sam Miller stated above, that the originator of the swim fin was Captain de Corlieu. My reference is none other than Dr. Hans Hass, in his book We Came From The Sea:
Such was also the case with the new diving method. This was not invented by scientists or by the technicians of the established diving 'industry' and they also did not arise from a special necessity. Modern skin-diving developed from fun and sport, it derived from a lust for adventure, from a delight in what is novel and from interest in the unknown.
The whole thing began with diving-goggles, such as had been long used by pear-fishers. In 1930 two Japanese, armed with such glasses, dived off Capri. About the same time Alexander Kramarenko in France took a pair of thes goggles and fitted them with small rubber bags te equalize the pressure. Probably others, of whom we know nothing, undertook other or similar experiments. They all experienced the marvel of seeing clear under water. They saw big, rather unsuspecting fish nearby. The idea of using spear or trident to waylay these fish came naturally enough.
The fathre of under-water hunting must be held to be the American author Guy Gilapatric. As far as I can recall, his imagination was fired by an American naval officer's account of how he had seen, during a voyage to the South Seas, Polynesians, holding a spear, dive into the sea. Gilpatric published in 1938 the first book to be devoted to the new sport. It was called The Complete Goggler. I look upon him as my master and he mentions one of my first adventures in his book.
The next contribution to the elaboration of the new method was made by Captain de Corlieu, a French naval officer, when he invented flippers for the feet. Soon the under-water hunters felt the need to perpetuate, for the benefit of others, what they themselves had seen and experienced. So we got under-water photography and under-water films. As a matter of fact, as early as the last century Wilhelm Bauer and Louis Boutain had managed to take under-water photographs, but of their achievement probably none of the young men engaged in the new sport knew anything. Each one, for himself, grappled with the technical difficulties in his own way. My first under-water camera dated from the autum of 1939 in my book Jagd unter Wasser ('Underwater Hunting') and my first colour photographs in 1942 in Fotojagd am Meeresgrund ('Hunting with the Camera on the Sea's Bottom').
Also inspired by the example of Guy Gilpatric, Jacques Yves Cousteau, Philippe Taillez and Frédéric Dumas formed a group at Toulon. They dived off the French coast and also off the island of Jerba in southern Tunisia. Perhaps it was because we in Vienna were so far off from the sea that we ventured, already before the late war, to plunge into the Caribbean.
However, what was still lacking was 'artificial gills'. In Toulon, as in Vienna, the talk was all about some appropriate breathing apparatus. Cousteau plumped for compressed air and devised, in collaboration with the technician Emile Gagnan, the 'aqualung' which is now used by divers all over the world. We on the other hand, as I have already mentioned, chose an oxgen apparatus, the first of which was turned out in 1942. As far as I know this was the earliest breathing apparatus (used by skin-divers) whose existence can be proved by published documents.
In much of the present-day diving literature the dangers presented by use of pure oxygen are much stressed. In opposition to this opinion I can say only this--we have experienced no trouble at all with such an apparatus (up to depths of ten fathoms) during more than two thousand under-water adventures. Similar resulats are also reported from Italy and South Africa. All the same, I recommend sporting divers to make use of compressed air appliances. They are reliable, easy to use and allow one to descend to considerably greater depths.
After the 1939-1945 war a whole industry developed to supply the needs of under-water sportsmen. Dozens of different masks, snorkels, fins and under-water guns came on the market. Diving-gear and under-water cameras advertised the new sport. Comic papers and cararet shows discovered the world under the sea. Diving clubs were founded. The first international hunting competitions were held...
Hass, Hans, We Come From The Sea, Doubleday and Company, 1959, pages 254-256)