We were on Enchantment in December.
My fiance wore a sport jacket the first smart/casual night and he looked just fine. However, he was the only one at our table of 10 wearing one, and the only one in our little area. We were on the 1st floor, in the center of the room in the row where the captain's table is.
He ended up not wearing any sport coats again.
I'm guessing each crowd on board might be a little different. We didn't even see that many tuxedos on formal night, so maybe we were with the "ultra casual" group!