The claim has yet to be substantiated by Israeli authorities. The country has a fleet of Dolphin-class submarines, but the news reports made clear that the individuals in Gaza were specifically referring to a marine mammal.
The fear of special agent dolphins, while amusing, is not without basis. The U.S. Navy, for example, maintains a detachment of dolphins and sea lions trained for reconnaissance. After Russia unilaterally annexed Crimea last year from Ukraine, authorities in Kiev demanded the return of combat dolphins that had been trained and quartered on the coast of the Black Sea peninsula.
And Israel, as Haaretz notes, has frequently been accused of using animals to carry out nefarious missions abroad:
Five years ago Egyptian authorities accused Israel of sending sharks to attack tourists off the Sinai beaches to harm Egypt's tourism industry, and two years later Sudan reported it had caught another Mossad spy — an eagle that had been tagged in Israel.
In 2013, the Lebanese Shiite organization Hezbollah claimed it had captured another Israeli "spy-eagle," and it broadcast images of supposed Israeli tags attached to the raptor.
Not to be outdone, in 2007, an Iranian state news agency reported the supposed "arrest" of 14 squirrels on espionage charges. The report was met with bemusement by many Iranians.
"I bet they were British squirrels, they are the most cunning," one Tehran street vendor quipped in an interview with NBC News.