The big itinerary decision is whether you want a weeklong cruise, visiting Hawaii islands exclusively, or a longer adventure, often from North America. The advantage of cruising from the mainland is that you possibly eliminate flights, but you could be adding as many as eight days at sea to the duration of the voyage (which some may consider a downside). Also remember that one-way airfares can be expensive if you're looking at a repositioning sailing.
When planning a cruise to Hawaii, there are basically three options:
Roundtrip from Honolulu: Norwegian Cruise Line's Pride of America is the only major U. S.-flagged ship sailing the Hawaiian islands year-round. Un-Cruise Adventures also offers seasonal sailings. Because ships registered to foreign countries must call on at least one international port, these are pretty much your only choices for seven-night, intra-island cruises. Itineraries are port intensive; Norwegian offers roundtrip sailings from Honolulu with overnights in Kauai and Maui, while Un-Cruise sails between the Big Island and Molokai, visiting more out-of-the-way destinations with opportunities for water sports and wildlife viewings.
Roundtrip from the U. S. Mexico or Canada: Carnival, Princess, Celebrity and Holland America offer seasonal Hawaii cruises roundtrip from various California ports (Los Angeles, San Diego or San Francisco) and Vancouver. These range in length from 15 to 18 nights, typically include a short stop in Ensenada to comply with legal regulations and have many sea days in a row at the beginning and end of the itinerary. Princess and Holland America also offer longer, roundtrip sailings that combine Mexico and Hawaii ports or venture farther into the South Pacific before coming back to California.
Repositioning: Another way to experience Hawaii is via one-way repositioning cruises. These may be between North America and Asia and/or Australia/New Zealand, or they could be somewhat shorter trips repositioning ships to and from Vancouver (and Alaska cruises) and back to southern California or even through the Panama Canal to the East Coast. Because these cruises only occur once or twice a year, when the ship is moving from one homeport to another, your options for dates will be limited. Most major cruise lines offer this type of repositioning sailing; if you want to visit Hawaii on a luxury cruise, look for longer world cruise or grand voyage segments, or Pacific crossings that may call in one or more Hawaiian ports.