Summarize:

Don't worry about dressing up for breakfast or lunch. Most people relax and lounge by the pool during the day on cruises, so break out your casual clothes for daytime activities. You should still make an effort to look presentable during the day. You wouldn't want to wear your pajamas to walk around the cruise ship. Wear jeans and a lightweight t-shirt or throw on a sundress. If you're going on a cruise somewhere warm, shorts paired with a blouse or t-shirt is an easy go-to casual outfit. If you're going somewhere cold, a heavy sweater with jeans or khakis works. You'll have a better time walking around the deck and relaxing if you're dressed in something comfortable. You may not be able to wear shorts into the dining room for dinner, so factor in time to change if you decide to wear them. Go with a pair of sneakers or sandals during the day, or a pair of flip-flops if you'll be swimming at the pool. Avoid shoes that might hurt your feet, like high heels and dress shoes. Wear a shirt and shorts or a dress over your bathing suit to get from your room to the pool area. When you leave the pool area, cover up again. Make sure you bring shoes for your walk to and from the pool area.
Wear casual clothes during the day. Dress in clothes you're comfortable in. Wear comfortable shoes you won't mind walking around in. Don't wear your bathing suit outside of the pool area.