To answer your actual question, country-specific requirements are here: https://www.celebritycruises.com/healthy-at-sea/travel
The U.S. requirements (separate from Alaska and Hawaii for some reason) state the following for unvaccinated guests:
Unvaccinated guests will be required to bring a negative PCR test conducted within three days of boarding. These guests will be given an additional COVID-19 Antigen test at the terminal prior to embarkation and on the day before disembarkation. The CDC has advised unvaccinated guests cannot be tested twice in one day as it may affect the accuracy of the test results, therefore the COVID-19 PCR test required before arriving at the terminal cannot be taken on boarding day. A registration link for required onsite testing at the terminal will be emailed separately.
Re: masking
While onboard, masks are optional but highly recommended for unvaccinated guests.