The current economic crisis (due to the state's excessive expenses compared to earnings resulting in deficits and a high accumulated public debt) in Greece has hit mainly the urban population and more particularly middle class. The chartering business depends on foreign tourists and as long as Greece remains an international tourist destination there will be boats available for charter at reasonable prices. On the other hand, islands is kind of remote backyard (the "countryside") and people living there have a low but constant income and their lives do not change dramatically over the years.
Conclusion: trouble and unrest are contained in one or two big cities in continental Greece (e.g. Athens) and you see little change because of the crisis in the insular periphery. Moreover, the currency in use is the euro which is stronger and less volatile than, say, the U.S. dollar ...