+1 on the zoom.

Like a camera, a zoom lens is more flexible for day to day use versus a fixed length. The ability to see a wider field then zoom in to whatever is interesting is very useful indeed.

With binoculars, you'll see numbers like 7x50 or 5-9x40. The first number or set of numbers refers to the magnification. So 7x50 = 7 times magnification whereas 5-9x40 means 5 to 9 times (zoom) magnification. The second number refers to the diameter of the lens. Generally bigger is better but that's only up to a point. A larger lens (50mm versus 40mm) means that the unit can gather more light and produce a brighter image for you to look at. A larger lens is obviously of greater use in lower light situations such as dawn / dusk or at night.

Something else to seriously consider is optical stabilization. Though expensive it's really quite useful. You'd be surprised at how hard it is to hold an image steady. Just standing on the ground and holding your breath, a high magnification image will move a fair bit. On a ship that's moving it will be harder still.

How much you spend on binoculars is a function of how much you intend to use them on the ship and afterwards as well. You can get cheap ones or you can spend a fortune. Broadly speaking you'll get what you pay for. Nice things cost money and optics definitely fall into this category!

Have fun on your cruise!