The clothing on the ships was definitely more dressy 10 years ago than it is now...probably due to luggage regulations from the airlines.
For formal night you'll see a few tux, but not many...a few military dress uniform, mostly suits. A few guys in a shirt and tie without jacket and a few in jeans.
Semi-formal is about the same, but the women will have more short dresses, fewer gowns. More people will be in "church clothes" and fewer really dressed up.
On shorter cruises, there is no "formal" or "semi-formal." It is listed as "dress up optional" and you'll see everything from full formal wear to shorts--no kidding!