wayne, the "deck"pants with a nice polo or and your teva's will be just fine for all your dinners with the exception of the one formal night on a 4-day cruise.
Norm is right, there is a special ambiance about the whole cruise on formal night, if you want to experience it all then i think his idea of renting a tux for the evening is a great idea, as for the shoes there are lots of super comfortable dress loafers that are soft as butter and feel like you are wearing slippers