Although the idea of webbed swimming gloves is attributable to Benjamin Franklin, it may be traced further back to another polymath, Leonardo da Vinci, whose sketch is reproduced below:
As for their use in underwater swimming, if you access the German diving equipment company Barakuda's 1961 catalogue at
http://www.sukellusmuseo.fi/esitteet/barakuda_1961.pdf
you'll find "Swim Gloves" featured on the eighth page. The resolution of the scan isn't good, but the "purpose" of these gloves is described thus: "Increases the speed of swimming by enlarging the area of the hand. When diving the glove enables the diver to turn and move more nimbly. At the same time they protect the hands and fingers against sharp objects (shells, sea urchins etc)". I recall seeing a pair in the sports department of the KDW (Kaufhaus des Westens) department store when I visited West Berlin in the mid-1970s, by which time they would have been around for at least a decade and a half.
Webbed swim gloves also appear on page 23 of Herb Taylor's
The Sport Diving Catalog, published in 1982 by St Martin's Press of New York. A company called Ocean Motion Research manufactured them in standard and custom sizes, the latter being available to anyone who sent in a tracing or Xerox copy of their hand spread.
Webbed swim gloves are available nowadays in at least three different materials: natural rubber, silicone and neoprene. They're sold here in Europe mainly as swim training or "learn-to-swim" aids. I've tried several kinds, including the Darkfins, and those made from natural rubber appear to provide the best flexibility and manual dexterity.
Some people will never see the point of webbed swimming gloves, others will swear by their effectiveness. Ultimately, it's up to the individual. I'm a strong believer in choice when it comes to underwater swimming gear and whatever matches my requirements - I always snorkel with traditional full-foot fins and a classic oval mask whatever the conditions - may not suit anybody else. We're all different and that's a good thing.