On our first cruise I thought formal meant tux and long gown, but we felt very over dressed once we got in the MDR. Now my husband wears a suit and I wear a nice shorter cocktail dress for formal and semi formal is just a button up and tie for him (no jacket). I found that folks were more dressed up on our longer cruises (Panama, Transatlantic, Baltic) but black tie wasn't standard on the 7 night Caribbeans.
There's no harm in dressing as up as you want, but you might not want to tell the husband that "everyone will be in a tux" like I did.