Margaritaville in Cozumel is downtown by the ferry pier. This is about 15 minutes by cab from Playa Mia. If your ship docks at the Langosta pier you can walk to Margaritaville. But if you are at International Pier you'll need a ride to Margaritaville. No matter where your ship docks - or perhaps anchors - you'll need a ride to Playa Mia.
There are always cabs waiting at Playa Mia and outside where the ships dock or their tenders moor. Trust me, you'll never have to wait to find a cab in Cozumel.