You might encoutner some cooler weather, especially toward the start of that cruise. The best strategy is to bring layers -- a couple sweatshirts or sweaters and a spring/fall jacket. You probably won't encounter winter weather, but some brisk fall days definitely are likely. You'll encounter warmer weather in the tropics the end of the cruise, so you also will want shorts, swimsuits, etc.
Of course, you'll need the so-called "resort casual" attire to wear on deck and evening attire in all three categories ("smart casual," semiformal, and formal). You can use the "smart casual" attire during the day in the cooler area and for sightseeing in Europe, where some tours may discourage shorts or other very casual styles.