For those "casual" nights a nice pair of "docker" type pants and a polo shirt, (shirt with a collar) will do just fine. You will only need a sports jacket if there is a semi casual, and a dark suit or tux for the "formal" nights. That should do it, enjoy your cruise!
Just got back from Alaska. I don't know what cruiseline you're travelling on, but for casual nights, khakis or cords with either a polo or casual dress shirt is fine for casual nights. If you have a semi formal night, you don't necessarily need a tie, but a jacket and maybe a turtleneck might work fine. You can always "recycle" the t-neck if its chilly at the glaciers. Hope this helps.