As long as you can say "Merida" a taxi driver will get you there and drop you off on the main square next to the cathedral. That's where you would also get a taxi to return. All you need to do is say "Progreso".
Once you get to Progreso you should be able to see the ship. Then you say, "Iremos al barque grande." He will probably expect another $2 for that.
If you would like, drop me an e-mail and I can type out the words that you'd need to know (show to cab driver) to get to Uxmal then back to Merida.