By strict definition, both formal and semi-formal attire is by "suggestion". While a good number of gents go for a tux on formal night, many also opt for a suit. And on semi formal night, you'll still see tuxes, suits, sport coats and no jackets. It really comes down to what makes you comfortable and what you enjoy, though this tends to get hotly debated on the boards, my opinion is that you shouldn't totally blow away the suggested dress code, but it shouldn't add stress to one of the most relaxing vacation experiences availble. And while some enjoy going the whole 9 yards, it certainly shouldn't ruin anyones experience if others choose not to.
