I should preface this by saying that cruise, to me, sounds like you are paying to be in a prison on water for 2 weeks or whatever, with occasional (land) reprieve.
That said, if you must, I think that for a cruise, you should dress comfortably. I am not a cruise person, and I happen to hate the heat and the sun, but if I had to deal with those things, I'd wear a lot of linen or linen blend pants, looser than usual, and probably some well fitted, but not fitted, linen and cotton/linen blend shirts. Shoes, I like slip on sneakers if I am just lounging around, and there are a lot of non-"streetwear" alternatives, like the woven ones by Bottega Veneta, but I think that this is one of the few times you have a legitimate excuse to wear boat shoes. Maybe a couple of unconstructed jackets for the evening. Lots of tans and browns, with some blues.