You’ll be traveling from island to island on regular ferries from Piraeus. These big boats are a guarantee of arrival on the islands. Greek ferries can sail at up to force 9/10, depending on their size. There can be a lot of wind at sea without you even realizing its effects on the islands. You just need to be in a sheltered cove or bay (or on the right side of the island) not to feel the effects.
Please note that in the Mediterranean, the weather can change, and boats can be delayed. We adapt to the circumstances. All boat timetables are subject to change.