I'd call the CVS at Post Office Square and make sure they are doing testing. Not all CVS's are doing proctored testing. I did a Google search (Covid test near Black Falcon Terminal) and came up with ARCpoint. This is on Drydock Ave and not far from the cruise terminal. I cannot figure out how to see if and where they have appointments without registering. You may want to try though: https://www.arcpointlabs.com/boston/request-an-appointment/
Finally, you could get a test at the airport. If you are only staying overnight in Boston you could get the test when you arrive at the airport. https://www.xprescheck.com/ search for Boston. Testing sites are in Terminal C (Jet Blue but connected to other terminals by shuttle bus) or in Terminal E (International flights).