The entire Greek coast is calm in the morning during this season BUT! Often in April, there are one or two strong winds and risk of thunderstorms in the evening and early night. They generally form over the mountains, so choose a night anchorage sheltered from winds coming from the nearest mountains. The water is not yet very warm, around 17 to 18°C, sometimes even less. Swimming requires great courage, but you can, like in the Atlantic, choose a spot with shallow sandy ground where the water is warmer or less cold.
The southeast coast of Corfu lends itself to this, though not particularly beautiful. Otherwise, look on the map. For example: Anti Paxi, Methoni with its large shallow sandy beach, the north of Pylos Bay, the unforgettable Elafonisos, and many others you’ll discover for yourself.