Tides of War by Marcus Turner. Part of the Drowned Earth Series. 

Maria may have survived the initial Rise, but the survivor encampment where she lives is now in the grip of famine. 

The crops are failing. Food stores are running low. Tensions are at an all-time high. Until one day when Maria discovers a dying man while out fishing. Upon regaining consciousness he tells her of an enormous floating 'lotus city', with enough food to feed the survivors many times over... except this city is reserved for the wealthy and powerful alone. Determined to right this terrible injustice, Maria forges an alliance with the other survivor communities, to take from the wealthy elite what they hoarded for themselves. 

The tides of war are rising, and the lotus cities will know the vengeance of all those left behind.