A cruise — a hotel on the sea, multiple port stops, entertainment on board — is the most comfortable form of travel.
What to pack?
Formal wear (for dinner), daily clothes, swimwear, sunscreen, passport, insurance policy — these are the essentials.
Money
Cards are mainly used on board. Small amounts of cash are useful at port stops. USD is generally accepted.
Seasickness
Possible in the open ocean. Motion sickness tablets can help. Usually the body adapts in 1-2 days.