When considering when to book, which can be a tricky choice, there are a few specifics that can guide your planning.
On Alaska cruises, for example, balcony cabins on newer ships usually sell out long before less desirable accommodations. Though these are more expensive, the demand is still high enough (and the supply small enough) that getting in early is a must.
Route matters, too. The Gulf Route between Vancouver and Anchorage is the most popular choice for Alaska cruises. Several lines offer on shore excursions as part of a package deal. Book as far in advance as possible, certainly no later than six months ahead for May-September departures.