Duct tape has 1001 uses on a cruise:
1/ Hemming and repairing clothing... suggest black tape
2/ repairs just about anything broken
3/ keeps that bulging coil inside the mattress, where it belongs
4/ keeps your stuff secure in rough seas
5/ fixes the thermostat (by covering the vent) or sealing beneath the door
6/ can be used to identify luggage (I suggest a huge black X)
7/ invaluable for impromptu costumes on theme nights
8/ put tape on the back of your sneakers and shoes to reduce friction/blisters
9/ if you tear your inseam on formal night, use black duct tape... people will never notice, or think you are wearing fashionable black underwear!
10/ to avoir getting lost on the ship, leave a trail of duct-tape pieces behind you. The crew will remember you forever....
11/ can be used to save seats (use "Police Line, Do Not Cross", or "Danger High Voltage" design)
12/ The roll can be used as a cup-holder (less stable as roll depletes)
13/ Lay horizontal strips on "saved" deck chairs adhesive side up. They're almost invisible on white chairs. Then everyone will know who the chair hogs are by their distinctive zebra-like pattern.
14/ Can be used to keep closet and bathroom doors from moving in high seas
15/ Tape allows you to play many more bingo cards than table space will allow. It can also prevent "hanging chads" from accidentally re-closing.
16/ Mark your cabin door with tape... they all look alike. Ditto "your" deck chair. The bigger the X the better. Then people will "know".
17/ Use a piece of tape across the arms of your sunglasses behind your head to keep them from falling off. Exercise care when removing.
18/ Tape your "Coconut Drink" to the railing, so it does not fall off and hit someone else's coconut.
19/ Tape your cutlery to the bottom of your plate, to free up your second hand for possibly more food in the buffet line...
20/ Can very quickly turn a plain white t-shirt into a striped one.
21/ Keeps deck furnishings from moving around on your balcony.
22/ Allows you to make "really, really good" towel animals. Show up your cabin steward.
23/ Can be used to darken the window if your curtains are too opaque.
24/ put tape on the bottom of your shoes, so you can waltz easily, even on carpet
25/ mark your muster station with a great big X, or even better, your initials, so you can be absolutely positive you are at the right one... it's a security thing. Ditto your life jackets, in case somebody should try to steal yours. Mark your territory.