Jeans are fine on colder climate cruises, do not think you will want them in the warmer islands. I have seen people turned away many times at the dining rooms with jeans on, so would not suggest wearing them there. You can of course wear them in the buffet dining area. Would think you would want to wear dress casual in the dining rooms except on formal nights. We usually skip formal nights and eat in the buffet dining room as it has the same food.