This is not "necessarily" true. You can still cruise, especially the Caribbean, with a passport that expires in six months or less. As long as it doesn't expire before or until after your cruise, you are OK.
The only problem is that some countries, especially those requiring visas, require that your passport be valid for six months at the time of entry to that country.
If you have at least 90 days before your cruise then it's worth it to get your passport renewed but if you are just cruising the Caribbean and your passport is still valid, then do it after you get home.
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me