Congratulations. This will be a nice honeymoon.
We will be on this cruise, also in the sky suites. This is our first time on the Mercury, but third cruise with Celebrity. There are some cabin pictures posted at http://www.geocities.com/mahdnc/Cabinpix.html
We enjoy the formal nights and have felt perfectly comfortable in a dark suit with shirt and tie (or even a silk type solid color tee shirt under the suit coat) Women will wear everything from nice pant suits to really nice cocktail and party dresses. It's not as "formal" (which means strictly black tie to me) as they will lead you to believe, although you will see some people dressed up that much and will be comfortable doing so if that's what you enjoy. Just dress up in what you are comfortable and feel dressed up in and you will be fine.
On our other two cruises we enjoy having breakfast served in the suite and a light casual lunch, either a salad buffet or burgers at the outdoor grill by the pool, but a nice lunch is served casual style in the main restaurant which is nice on the at-sea days. We usually found we went back to the ship to eat, even on our port days - the food on celebrity is just that good.
We are doing the train shore excursion from Skagway, but understand from others on this discussion that sitka and ketchikan can be explored on your own. There is also some discussion about booking the whale watching excursions independently of celebrity. We have usually found on our other cruises that we like to pre-book a major shore excursion or two if its something we really want to do and then book additional smaller excursions a day or two in advance on the cruise.
If you have problems with motion sickness, you might purchase a pair of sea-bands (or C-Bands) from your pharmacy at home. These are elastic wrist bands with a bead sewn in that applies pressure to an acupunture point above your wrists. I have absolutely no problem with this (knock on wood), but my partner gets sick on a drive across town and these worked for him. There are no side effects. They can always give you medicine in the ships clinic if you get really sick and they sell the drugs also on board. The ships medical staffs are very qualified and friendly and usually underutilized and happy to help you out. Also - with a high center room on the ship, like the sky suites, the roll of the ship is minimized and you'll feel less motion than below decks and forward or aft.
Plan on a good time - maybe we'll see you on the ship.