'Chartering bareboat' is the nautical equivalent of 'trekking without a guide'. A bareboat charter without a local professional skipper means that you will skipper the yacht with some help from your crew, and you will be the person in charge in port or at sea until the check-out of the yacht.
Bareboat rentals in Greece require onboard one (1) valid offshore sailing certificate from one competent and experienced skipper, and from a second person on board a signed official declaration on spot, proving his knowledge to sail offshore a sailing boat (or catamaran) under sails.