If you are travelling with P & O, or Cunard, check their dress code.
Both lines have specific 'formal nights' when evening wear is required for passengers.
Of course, there are those who don't adhere to the codes, but they are seriously in the minority, so don't be left feeling uncomfortable!
Obviously the weather is warm, so pack as you would for a Med land holiday - but take a few light cardies - the air-conditioning can sometimes be a bit fierce on board!
There are laundries on board, and a laundry service, so don't worry about washing.
Pack sensible lightweight walking shoes, terrain varies different places, so make sure you are equipped for occasional rough ground.
The rule is, be comfortable.
Have a great time!