Speaking to “recent experience”, we are on the Sky right now. Southampton to PE. 27 days. (At this moment alongside at Grand Turk.)
On formal nights (9 or so), we have observed all manner of dress in the MDR ranging from tux and sparkly gowns to jeans and short sleeved collared shirts. The split was fairly even but overall tended a bit toward casual over tux/sparkly.
On non-formal nights, everything from shorts/tee shirts to formal wear can be found in the MDR.
We are in Club/Reserve. This area tended to be a bit dressier than the main section of the MDR but yet there were jeans and short sleeved shirts throughout on both formal and non-formal nights.
All of this said, you will not feel out of place regardless of your dress choice.
Be comfortable however you define this.