A 3 way plug so can plug a few things in at once in your room.
A portable CD player and speakers/ headphones or Ipod with favorite music
Bonine for seasickness - over the counter in Walmart. The Gulf can get pretty rough for no reason so the first day out and last back can get (not always) rough.
Underwater camera if intend to snorkel.
suction cup hooks that you can use in cabin to hang stuff on.
Lysol to spray down cabin - I spray everything! First sometimes you can get a musty smelling cabin or one that kind of smells like smoke and Lysol cuts all that and makes it smell fresh. Also they don't wash the top blanket (bedspread) between passengers so I spray that. Also the Norwalk Virus can run through a ship fast so I make sure I spray the ramote too.
Small travel size of that germ killer stuff for tours etc.
Always wash hands - only takes one person with the flu to touch elevator buttons and handrails to make a lot of people sick in a confined space.
plastic ziplock bags - always handy. Sometimes you may even want to pack some stuff from the buffet to take off ship. I don't think you are suppose to but people do. I take the box cereal from the buffet and feed to the fish at whatever beach I'm at. Some people take rolls in which you need the plastic bags.
One bottle of water that you can refill and take off the ship. The water on board is fine to drink but not in Mexico so I fill up a waterbottle with ship watre to carry with me on excursions. They want a ton of money $3 per bottle on board.
In Playa the jeep tour was very highly rated tour by everyone that went - our backs were too sore from our tour in Cozumel. If you like something different in Cozumel they have the speed boat (powerboat) tour where there are two people boats - you or partner drives and you go 45 minutes in the boat (8 feet long) in open ocean to a deserted beach for a buffet, swimming, volleyball. Then 45 minutes back with you driving boat. HUBBY LOVE THIS!!! He said it was worth every penny of the $90 a person it cost. The boats are only 8 feet long and there were some 2-4 foot waves and he was "surfing" the wake of the boat in front - it was exhilerating and a few times scary with him driving!!! You can pass or travel outside the wake of the other boat but hubby was having too much fun!!
Our backs hurt the next day though!!! Debbie