You'll have to pay the Greek cruising tax online, before entering:
https://www.aade.gr/polites/etepaiYou also need a crew list onboard and update it when the crew (= visitors) change. Greeks are very keen on busting unlicensed charter, but it's not a problem if the owners are on board and you declare visitors to be crew. Port Police (= Hellenic Coastguard) does spot checks, but their efforts vary. More information on Noonsite.
If you want to winter in the water, there's popular liveaboard marinas in Gouvia (Korfu), Lefkas and Preveza. Book early. Most people who aren't spending time onboard in winter probably leave their boat on the hard. There's a
huge hardstanding area on the Aktio peninsula opposite Preveza with three yards to haul and put up your boat. Also several smaller ones all over the place.
For a pilot book, the most popular one by far is Rod Heikell's Greek Waters Pilot (Imray) and it served us pretty well. Plus the NoForeignLand online community and the comments on Navily with tips from other cruisers.
In comparison to the Croats, the Greeks are much more relaxed and friendly, at least on average. As most of them are boaters themselves, they understand boats like no other and you can anchor pretty much anywhere you like without being bothered, and often there's a dinghy dock built by an enterprising restaurant owner. Town quays used to be free or cheap or tied to a restaurant visit, but this is slowly changing as capitalism takes over. You can still find them though. There's even some abandoned marinas where you can moor for free if you find a spot, but it's often a bit chaotic. Beware of staying on town quays in strong winds. Damage from weather or more often careless other boaters is extremely common. It's often smarter to anchor off and visit by dinghy or stay on the boat and watch the show, especially in the northern Ionian, which is very popular with absolute beginners chartering first time. Hilarious when you're watching at anchor, less so if they're trying to get into the berth next to your boat.
Don't mess with ferries, fishermen and underwater cables. Those three supply the islands and are more important than you, no matter the official rules