Depending on where you are going and which islands it might be quite hot but you will only be ashore for up to max 9 hours and the ship is air con and once on the move you will get the breeze associated with a moving ship unless you find a nice little corner which is sheltered. No I think jealousy is playing apart and even if it were hot you are going to take the right precautions so enjoy your cruise.
No definitely not a mistake, I am no sun worshiper in fact I do not sun bathe and I really enjoyed our last Cruise in the Med and that was July 2008. You can always find shaded areas outside on the ship where you can relax. A dip in the pool, lunch around 1.30pm a cool beer, go and enjoy. .
Yes, it will be hot, but have these people never heard of aircon? In really hot weather you can always retreat indoors mid-day for a siesta. Shade is provided on ships as are cold drinks. As Blythe says the breeze on a moving ship helps. Have a lovely holiday and tell your jealous friends all about it when you get back.