The only time life jackets are worn is during general quarters. Makes sense.
Deck personnel will also wear life jackets during special operations such as sea and anchor detail or unreps
Nope they are required in rough seas as well. I served on a WW II tanker that had a lower deck barely above water level and I have see green water above the second deck the only way you went on deck was with floatation gear and lashed to safety lines. Other wise cruise along like any other cruz liner.