Just because your going on an exotic cruise to the Caribbean to soak up all that heat and sunshine it doesn’t mean you only have to pack shorts and t-shirts! First of all the ships air conditioning can be very cold and when walking around inside you will deffenetely need something warm. That also applies to the open decks because at night when in the open sea it can get pretty windy.
Secondly just because there a nice blue sky and sun from the online pictures doesn’t mean it will be like that when your cruise ship arrives into the destination! Rain and grey clouds can be popular so be prepared.