Hi there, I wouldn't worry, 'hot' is a very subjective value, what is hot to someone from Northern America may be quite cool to someone from say Florida or the Caribbean.
Temerpatures are also always cooler on the ships as you tend to get a sea breeze across the open decks.
Take a look at Gate 1 Travel - Europe Average Temperatures
it has the average temperatures of the major cities month-by-month, I am sure it will resolve any concerns.
Hope this is of help, and I wish you a great time
Last: Oceania Nautica Aug 2016
Next: Cruise Maritime Magellan Nov 2016
Then: P&O Oceana December 2016
Then: NCL Jade August 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey