I used to think I would hate cruises, but I kind of like them now. DP is a huge fan of them.
On sea days you may be surprised at how cold it is on deck, even if it's sunny, because when the ship is steaming along at 20 knots, that's like a 20-knot wind coming from the bow (unless there is a real 20-knot wind coming from the stern, in which case the ship uses less fuel and you get a nice calm day!). It took me a while to work out why it was always windy, and I have a STEM degree!
Everything about the indoor environment is ruthlessly controlled by the air conditioning. There are no open windows. But it's not stuffy. You can walk around in a thin long-sleeved top the whole time, but you won't be chilly in a short-sleeved top in the restaurant. (Some ships have dress codes for dinner, but you can always go to the buffet if you CBA to get changed.)
Don't hesitate to ask for substitutions or extras in the restaurant. If you want three starters and no main, or two mains, then order that. It's better than having half/full board in a hotel where you just have a set menu and they have an eye on the price. (Obviously the restaurant cooks have a budget, but your extra food won't break it.) But if the food is good, which it very often it, then watch out for the calories!
It's important (and just generally nice, of course) to be respectful to the staff, especially your cabin attendant. Even if "tips are included" or "a service charge is included in your final bill", do consider tipping the cabin attendant on your last full day — and I say this as someone who hates tipping in general. Most of them are very good, and they come from poor countries where your tip will go a long way.
The ship's organised excursions can be poor value for money. We usually go into the local town and explore, maybe take a local harbour or canal tour (you're always in a town on the water!). But be scrupulous about respecting the all-aboard time. Take your passports with you whenever you go ashore, partly because you may need it to get back on the boat and partly because you will look silly otherwise if you miss the boat — which can happen for "good" reasons such as a medical emergency, as well as "bad" ones like getting the wrong bus back to the port.