Informal is just a step below formal normally. People aren't sure because informal and casual often mean the same thing. Formal would be black tie/tux required and informal would mean suit and tie. Casual is sport coat. With dress codes all mixed up no one really pays much attention - even on ship.

VOYAGER:
MC to FLL 11/07
MC/Dover 6/06
MARINER:
BCN / Ven 10/11
Alaska 2001|2006
Panama Canal 2003
NAV
Caribbean 11/06
Bermuda|New Eng/Canada 6/04
PG 5/05
OCEANIA
Nautica |Ist/Athens 6/07
Regatta|Baltics 6/08
Riviera 2013 Crossing -Istanbul to Miami