Looking an my navigators from the 2/13-2/17 Wonder Cruise here is the follwing information.
Sunday- Nothing really listed for dress
Monday- "Evening attire: Casual or Pirate Attire"
Tuesday- "Evening attire: Dress up night"
Wednesday- "Evening attire: Cruise Casual"
One could take "Dress up night" as Formal, but saw many people very casual still. I dressed up for Tritons dinner, even though it wasnt "Formal night" but seemed like the right place to dressup. Seemed many people did the same.
Hope this helps some.