It should be warm, but no one can predict the weather this far ahead. Last year on 30th September Athens was basking in 84 degrees and on the same day Istanbul was 75 degrees so it looks good although when at sea it will be a lot cooler with the movement of the ship and if any prevailing winds about. Having a lightweight jacket would do no harm if in an evening after dinner you wanted to stroll around the ship.
I did these dates in the Greek Isles last year and the weather jumped from very sunny one day to pouring with rain the next. But even though we had rain it was still hot. We just took light rain jackets, but beware, most streets in Greece are marble and the pavements/roads are very slippy when raining. My dad aged 74 actually fell and injured himself. Hope this helps.