It's across the board and somewhat depended on the place. Riviera, being the adult beach/pool area, was mostly people in swimsuits. People seemed to dress up more for Tabemasu, and more for dinner in general. Shorts were still frequently seen at dinner, so I don't think there's any pressure to wear slacks or a dress unless you want to.

You can take lunch at Velaa in the beach loungers instead my of a regular table, so pretty casual. They redo the table set up in the evenings and it has a nicer feel then.

As anywhere, some people dressed to impress most of the time, but it's also a place where people are diving and doing other watersports and dining right before/after. I didn't feel awkward at breakfast or lunch if I was going/coming from diving and was more dressed down than some others 😄, but I did generally put extra effort into dinner appearance