Instead of complying with the rules, Mr. Melnyk went rogue. He committed himself to his fishing method, attracting others with a penchant for the extreme. Today, they're challenging what it means to fish.

A recent YouTube video shows a man swimming in Florida with a mask, snorkel and flyrod, catching a tarpon. ZeeBaaS, a fishing-gear company in Stratford, Conn., recently added a section to its product line and website devoted to the activity. Cable channels, like National Geographic, have featured it. Enthusiasts sharing tales on websites hail from the U.K., Turkey and South Africa.

"There used to be a whole bunch of people against this concept," says Mr. Melnyk, whose right shoulder is covered with a tattoo of a skeleton, holding up a massive skeleton of a fish. "But there's a new generation of fishermen out there."