Yes, you are correct! There are no formal nights, theme nights or dress-up nights. This trip is completely casual. The best shoes to bring are toe-protected Keens or Tevas for wet landings, hikes and days on the beach, tennis shoes or another pair of dry shoes in case your other pair is wet for around the ship, flip flops . . .
The best clothing to bring are shorts/shirts that can be hand washed and quick drying - seriously! ExOfficio and REI have some good items, and you may be able to pick up a sale on them after the holidays! There is no laundry service on board, so it's important you bring as little clothing as possible that you can wash in your cabins. I LOVE the Patagonia and ExOfficio quick drying underwear (3 pairs are plenty).
The days are warm, the evenings may have a slight chill. A light wind-jacket for the deck in case we get an early morning call of a whale sighting :-) or want to watch the sunset in the evening, T-shirts, micro-fiber shirts with and without sleeves that dry quickly (sleeves for evenings and to protect you from the sun during the day), zip-off pants so you have shorts in the day, and long pants at night - shorts that quick-dry (we do a good amount of kayaking as well) :-), bathing suit to snorkel and swim with the sea lions (we provide a wet suit) . . .
We wil be sending you a pre-departure booklet prior to the trip that will outline all of these tips and more.
Happy Holidays! Jill