Definitely casual! There are both sit-down and buffet options for breakfast and lunch, plus room service is always an option. There are no assigned seating times, just show up to the restaurant when they are serving. And shorts are fine. You could even wear a bathing suit with a cover-up during the day at the buffets.
They definitely will not allow bathing suits, with or without coverups, into the dining room that is serving the sit down lunch. On our cruise this past week, I saw a family in bathing suits and coverups turned away from Triton's at lunch.
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