When we went on the Wonder cruise in 2001, my husband wore a jacket and tie the first night at Tritons (we had two rotations at that restaurant). Personally since it is still kinda warm, I think "resort casual" is right down the middle. Resort casual consists of a collared shirt with dress khakis/slacks and comfortable shoes (not sandels, but could be clean tennis shoes). Think Tiger Woods on the golf course. This works well because you won't stick to the seat as you would wearing shorts.
I wore a comfortable cropped pantsuit. I don't like to wear pants, shorts or skirts. I'm in capri's or cropped pants 90% of the time. My pantsuits were made of silk. Inexpensive and comfortable, but beautiful. I'm not a skinny minnie; I purchased the outfits at Catherines.
~Hope that helps.