You may have difficulty getting the prescription filled at an outside pharmacy if you are under isolation.
Also, if they do not have the Paxlovid on the ship, most ship physicians are not licensed in every state and country the ship sails to. Some places will allow outside prescriptions, but some will not.
As was mentioned before, you should only get Paxlovid if you are high risk. For others there is very little if any benefit and it has some nasty side effects and increased risk of rebound COVID, as well as the very high expense. If you are such high risk, you will likely be disembarked at the next port, to a hospital or a hotel under isolation. Remember too, that your insurance may not cover Paxlovid if you do not meet the indications for it. Likewise if you just get the medication 'just in case' prior to a cruise without a positive diagnosis.
Asinine perhaps, but it makes sense to me.