In the summer Crete is quite crowded usually. I spent a nice week there and most of the resorts in the north are too lively for me. There are many young tourists who party all night long. So try to choose accommodation carefully.
About the diving... Well, many of the dive centers prefer organizing trips for discovery divers. They had a huge boat, they take a lot of beginners to a boring, shallow bay, and sadly the certified divers too. So I'd recommend to look for dive centers in the area where you stay, but check their websites: if they have too large boat usually it means they don't care too much about the needs of certified divers.
A few years ago I dived with the
Atlantis in Panormo- the owner said, they have another dive center which has more discovery divers, but in Panormo they usually do normal dives. I liked the place and dives too.
We spent a day in
Istron bay where we dived with the Aqualand. It was OK, and there was only one large hotel in the bay, so maybe it's a little better for relaxing.
I know some guys who said the southern side of Crete is much more relaxing, but we made just a short trip there, I don't know anything about possibilities for staying and diving.
