On short cruises like yours, if there is a formal night, most people will not be dressed up fancy. A few may wear a tux or a suit, but most will be in slacks and a shirt. Usually the dress code for the dining room in the evening is no shorts, even on casual nights - but again it is possible the dining room staff will look the other way. I would plan on throwing a pair of khakis or decent slacks in your bag - topped with a button up or a polo.

I will tell you it tends to be cool inside the ship especially at night.

The ship you are on doesn't really have a specialty restaurant as an alternative. But the buffet is always on option.