I'm seeing some older rules, from when cruise lines hadn't been shut down completely yet. I'm also seeing a new rule about masks that was implemented just 2 days ago.
At least not that I've been able to see on the CDC website under news.
Here's the CDC's page, the top new thing dated May 5 is to say that there are new guidelines that allow cruise ships to start simulated voyages, to test out the mechanics of getting everyone sailing again.
Cruise Ship Guidance
| Quarantine | CDC
If you follow the various links, you learn that all the OTHER rules were implemented when people were still totally unprotected on ships, and some cruiselines were still taking no precautions at all.
I'm not finding anything new other than the guideline for the simulated voyages (which means - not actual voyages).