I interned with a cruise line while in college. I kept on board ship and currently in my boat a very small waterproof bag with a set of rain pants and a hooded jacket (rain, wind, and sun protection), a boonie style hat, good wrap around sunglasses, a weeks worth of prescription meds, water packets or water bottle(s), SPF 45 water use sun screen, space blanket now replaced with AMK 2 person heat sheet(s), a book or 2 (keep the mind occupied and emergency TP), camping TP, plastic whistle, small LED light, a small modified military fishing kit with extra 50 pound line, FAK, a SAK and a few energy bars. .I'm not sure if a SAK is allowed on cruise ships at this time.

The AMK PSP and SOL kits weren't available when I had the cruise ship bag, but they are part of my boat bag.