Casual for me - shorts and shirt is good. I don't think I'd wear a tank top. I don't think you will see very many people dressed up to board the ship. Usually the dining room doesn't enforce the "no shorts" policy (RCI/Carnival/probably others but not sure) on the first evening since they know the luggage doesn't always arrive in time for you to get ready for dinner.
However, I am not familiar with the Harmony (cruise line/destination). If this is a European cruise, I think the rules may be a little different. I've heard (haven't been to Europe yet) that the European cruises are dressier than their American/Caribbean counterparts.
being that Crystal Harmony is considered and "upscale" cruise line i would go with a nice pair of linen or cotton pants a polo-type or button-down short sleeve shirt or blouse for your wife a nice pair of casual-comfy shoes and maybe a sweater or blazer in case its chilly. Capris and a sweater-twin-set would also be fine for your wife. Think resort-country club casual wear.
Wear whatever you're comfortable traveling in. You'll see EVERYTHING boarding the ship, from shorts and tshirts to ladies in floppy hats and dark glasses, full makeup and nails as long as their heels are high. You'll see couples dressed ALIKE... I don't know why, but some couples we saw on the Serenity this summer seemed to like matching outfits. It's true that Med cruises can be dressier, but not everyone follows that trend. If you're someone that likes to dress... go for it. If you prefer casual, that's fine. When we flew to Rome in July we purchased a transfer to the ship. We were put on a bus with 5 other couples. Those were the only people we boarded with. The only people that will think less of you for not dressing up will be the people dressed to kill. Personally, that doesn't bother me.