A change of clothes for each day you're on the ship. (Jacket/Shirt/Tie/or equivalent for Wardroom, smart casual otherwise).
Warm coat, washgear/toiletries, bath towel, flip-flops. All protective clothing is provided by the unit. Recommend cash (no cashpoints) approx a tenner for incidental spends for each day (max) should suffice.
For a definitive list ask the person inviting you.
You would normally wear the civilian equivalent of the service uniform which is dictated by what the ship is doing operationally, where it is globally & what time of day it is.
If they are all wearing overalls during the day, you will too. If not wearing overalls you may be loaned Fire Resistant shirt & trousers (No4's) or wear smart casual civilian attire (shirt, collar attached, trousers & shoes NOT trainers) If they are wearing "night rig" in the wardroom & you're in the wardroom, then lounge suit(or female equivalent), shirt & tie is appropriate.
Again, best bet is speak to the person inviting you for definitive guidance.
To be fair I go on two to four cruises a year where some evenings are black tie affairs and most other evenings require smart casual dress. Time and again MEN are turned away after arriving to dine dressed in shorts and t-shirt, and these are mostly from the older generation. Interestingly the Chicogiz generation seem better educated than their older peers about dress codes! :roll: