If you rent it online from a big company you will need one for sure. But if you rent when you are there from a local company it could be possible just to use cash. Anyways they use the credit card only as a "insurance" in case of problems. You usually can pay the rent with cash.
If i can give you a suggestion I would prefer a car to go around in Crete. The island is very big and has some hight mountain (around 1000 meter) so a scooter could be quite unconfortable. I would consider it only for very short trip.
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