I just got off that same ship in Thailand. We loved it!
For breakfast and lunch in the dining room, you can wear very casual clothes like shorts. No swimsuits though. And you always need shoes.
Men: long pants, shirt with collar and sleeves. After Day 1 dinner, men not meeting these rules will be sent back to change.
Women: no shorts. Nothing else is stated in the rules but I assume women would also be sent to change if a dinner outfit included shorts.
After beach/swim: walk around the ship with a cover-up or sarong over suit, both men & women. Don't walk / sit all over in wet beach shoes or towels.
Other than those few rules, you can wear whatever. The cruise was not "formal". All people did look nice though, meaning their ratty stuff was saved for beach lounging, if indeed they even HAD any ratty stuff with them.
Our biggest problem was bringing clothes to meet the dinner dining room requirements. Turns out many people just wore one or two outfits anyway, since no one wanted to overpack. There were many comfortable outfits seen and many fun ones too.