657 (new?) islands discovered

They found 657 more islands than in 2001. Mostly barrier islands. New ones? In a few cases: Barrier islands are sandy and form and erode away. But mostly existing islands were missed due to low resolution of photos and some science - see below.
Live Science

... The newly identified barrier islands didn't miraculously appear in the last decade, said study team member Matthew L. Stutz of Meredith. They've long existed but were overlooked or misclassified in past surveys. 

Previously, for instance, scientists believed barrier islands couldn't exist in locations with seasonal tides of more than 13 feet (4 meters). Yet the new survey identifies the world's longest chain of barrier islands along a stretch of the equatorial coast of Brazil, where spring tides reach 23 feet (7 meters). 

The 54-island chain extends 355 miles(571 kilometers) along the fringe of a mangrove forest south of the mouth of the Amazon River. Past surveys didn't recognize it as a barrier island coast partly because older, low-resolution satellite images didn't show a clear separation between the islands and mangrove, Stutz says, but also because the chain didn't match the wave-tide criteria used to classify barrier islands in the United States, where most studies have been conducted. 

Scientists failed to consider that supplies of replenishing sand are so plentiful along the equatorial Brazilian coast that they can compensate for the erosion caused by higher spring tides.

The photo: Science of barrier islands from U Texas Bureau of Economic Geology. Click to enlarge.