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
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