Well, it not always like that.
This is the demo (Just joking) !
There are heavy seas too, and cruising during the winter can be quite cold and you have to wear thermals, drysuits etc.
Generally speaking you don't need any special clothes from May to Octomber. Your swimming costume is fine.
During the winter and especially December to March is a different story.
Simply we have different weather conditions. To be honest I enjoy ribing both during the summer and winter periods.