Formal is Suit and Tie for men,
as for 'part of the experience' .....on Cunard Line, I reckon about 99% of passengers dress correctly (the others eat burgers in the buffet),
some women get completely absorbed by it and wear complicated ball gowns, but most just dress to look nice and smart.
PS- I am not a suit wearer or a tie wearer at all, but I do for formal nights.....
