Actually that is exactly the cruise I took in Aug of last year.. I am going again in July of 05... I also spend 4 days in NY first....
I was always COLD! The ships are air conditioned and I was always freezing! So I suggest a nice silk or similar cardigan sweater, black will go with everything..
I ended up buying one in NY before I got on the ship, cuz I was cold even in the hotel in NY...
I personally would bring a paid of shorts or 2 for the trips off the ship... but otherwise I'd bring jeans (if your a jeans gal, I know I am) for shore trips and day time wear.. And short sleeved shirts... Then maybe a casual zip up sweats/valour style jacket, just incase.. EVERY evening will be "resort casual" which means sun dresses, light slacks, blouses, skirts ect.. And for formal evening your dress sounds completely appropriate, LOTS of people will wear the same type of thing.. you may however have 2 formal nights... Check out a department store like Dillards in Jan for SUPER sales on formal wear.. But yes a prom dress would be exactly right. As for men, a lot of ships say men wear jackets every night and suits/tuxes for formal night, which is a load of crap
Most men had nice work style dress shirt or short sleeved blouses on the informal nights, a few in jackets w/o ties... and the formal nights they wore suits.. I only saw 1 man out of the entire group in a tux.
I would also bring a skirt or two and an outfit or two that fits on the looser side (elastic waist) to wear toward the end of your trip, for the 2 lbs average a day you gain
Have fun Terri