it's a combination of both - luckily most cruises have at least 2 formal nights!
for my upcoming one, i will be bringing both a long, lean black evening gown and a fun, red, cocktail dress that hits just above the knees. one side is a strap over my shoulder, the other side is a bat wing. hoping to find a pair of dressy silver heels (not buying just for cruise but want one anyway) to go with both. i also have a formal blazer that's nicely tailored to my body. i will bring it in case it's a bit chilly.