Many ocean cruises are booking out a year in advance, especially popular, holiday, special event or limited availability itineraries. For example, Disney Alaska, departures out of San Francisco and itineraries that include the (British) Open are booked quickly. Many world cruises sell out within a couple of weeks being published.
Certainly there are destinations and itineraries with availability close to departure. The larger the ship, the greater the chance of availability, especially if your travel criteria is flexible and you live close to or can get to a port quickly. However, your ideal or preferred cabin, departure date and/or itinerary of choice probably will not be available. And keep in mind many cruise lines no longer discount last minute fares.
So start your planning (and booking) early to ensure a most wonderful cruise experience!