A word of advice:
even though the law has changed recently with regards to diving and where is is allowed (and where it is not) I would be very cautious diving without a centre. (obviously i don't know if you where planning to do your own thing or not, but there you go) Things have always been restrictive when it comes to scuba in greece due to archaeology and the coast guard / police are not kidding about catching people and fines. That law has changed but as usual (and i'm embarrased to admit) things in my country are never quick to be enforced properly. My advice is dive with a centre and get advice from them if you wanna do your own thing about who you might need to notify before jumping in and make sure you have your quals with you if you go off on your own.
Also bear in mind that night diving may be heavily restricted. Also you may be required to carry a blob when diving (day or night). If you have a dsmb of your own, take it with you (better to use kit that you know than kit that you don't)
Words of caution aside, i hear the diving in Crete is lovely. There was an article in Diver recently about diving there
link
HTH
Have a great time