Hi,
It will be hot, hot, hot...so take your shorts...and cool tops. Also while in Cozumel you can find some really nice cool cotton outfits in town at the markets. I love Cozumel...it is my favorite place for shopping in the Yucatan.
Dressing for formal nights isn't as class conscience on Carnival's Fantasy ship and this ship is the sister ship. There will be some people wearing tuxes and gowns, but I've found that there aren't all that many. A suit for a guy and nice dress or pantsuit for ladies will do.
If the Sensation is like the Fantasy, she will have a guest laundry onboard, but make sure you get down there quickly to do whatever pressing and cleaning you have to do to get ready for formal night. It gets crowded and people line up in the hallway.
I have not seen hairdryers in the cabins and there are no irons onboard for safety reasons...except in the laundry room.
I am assuming that this ship has the same features as the Fantasy...and I know what happens when one assumes. So it might be best to call Carnival and get the skinny on it from them.