The thing about Greece is that the market is very fragmented and you will find various local companies on the big islands. You can also go for one of the so-called 'sharing economy' platforms if you are into that kind of thing, I used to rent out my
boat via
Click&Boat, they are quite decent and have many
boats in Greece. I think there is also a Greek platform, it's called incredible if I remember well. The good thing about platforms is that you can check
reviews, sort of Airbnb. Where are you sailing btw? Would be able to give you tips re moorings etc if you are sailing in the Aegean!