The Little Hell Gate Inlet is all that remains of the channel that once separated Randall's and Wards Islands. (To get a sense of how the islands have changed over the years, compare these aerial images from 1924, 1951, and 2012.) The eastern half of the inlet has been restored as the "fully functional salt marsh" you see above. The footbridge in the center of the photo, spanning the inlet, is part of a recreational path that passes through the marsh.
If you've been wondering why Randall's Island has an apostrophe in its name and Wards Island doesn't, I don't have a great answer for you. Both islands are named after former owners, so it seems apostrophes would be in order in both cases to indicate that the names are possessive. But possessive apostrophes have long been frowned upon by the US Board on Geographic Names, setting an oft-followed precedent for their removal from place names across the country. NYC is inconsistent with its apostrophes (although you'll almost never see one in a street name), and you can find both Randall's and Wards Islands written with and without apostrophes on different city signs and maps. But for some reason, the city government is more often than not inclined to include the apostrophe in Randall's and omit it from Wards. (See, for example, Randall's Island Park and Wards Island Park.)