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The weather is starting to warm up by then but you would be well advised to take a sweatshirt or something warm and something waterproof as well. Storm are not unknown in the Mediterranean in April and May. If you are sailing near an major river outlet the sea can be quite cold still even as far south as places like Corfu. Having said all that you should be able to get some sun bathing done.....regards...Neil (Sorry I don't know your actual itinerary but it does sound like Greece.