Standard day wear is usually shorts and a t-shirt or other top. Or Swimsuit and cover up.
My standard day wear is swimsuit or shorts and a t-shirt.
Some folks do wear a polo shirt and pants and shorts but that is up to them.
A cover up for women and a shirt for men is required in the buffet. However, on my last Carnival cruise I noticed a lot of people in bathing suits and no shirt or bikinis in the buffet. I guess they weren't enforcing the rules.
Dress how you feel comfortable. "Within reason."