Passports, TSA PreCheck and other mysteries of the universe....
1. From the TSA Website... you can get a KTN without a passport, however the required documentation may be trying. Might as well get the passport...
2. Passports and ships, even planned US domestic-only trips... from long personal experience with ships (and airplanes), they don't always go where (or when) as planned) for countless reasons. Having a valid passport can significantly ease your "revised" travel plans. There will not always be a convenient Embassy or Attache Office if you wind up in some little port in some moderately unfriendly country. Come to think of it, the same applies for airplanes when the itinerary is outside of the continental US or going to a "passport not required" destination.
3. Please apply judgmental filter: IMHO, all adults should possess a valid passport. If you are entertaining any cruise ship travel, it should be a mandatory personal item regardless of planned itinerary.
4. More judgy opinion: If you're balking at the couple of hours of prep work and the $160 fee ($16/yr... or about four adult cups of coffee at Starbucks) perhaps you should re-think cruising.
5. While you're at it... one of the best investments you can make is to get the Global Entry. Price is $100 (vs $85 for the PreCheck) AND, PreCheck is included. You will endure some schedule shenanigans to get the interview, but our planned six month wait was rapidly whittled down to five weeks as appointment openings came up. It's really easy if you're returning from international travel, just go to the GE kiosk/office, do the interview (fog the mirror) and your card appears in the mail.
6. When ordering your passport, get the additional page version... it lasts longer. Some places require a minimum of two unused visa pages or... you don't get to come in.
7. The answer for the mysteries of the universe is "42".