Not all taxi drivers speak English well and many are from Cancun and will not know Casa Roca Caribe without directions...so you will need to be able to handle that.
Directions in English: "We are going to a house very close to the Mango Cafe. When you approach Mango Cafe (about 2 miles south of the ferry, you will see Mango Cafe on your right) ...go straight for one more block and take a left on street Emiliano Zapata (there is NO street sign there). Casa Roca Caribe is up the block and is a two story house on the right. it is hard to miss, there is a big blue sign on the house that says pretty clearly "Jimbo and Karen's Casa Roca Caribe"...so you can be quite certain which house is ours.
Directions in Spanish: "Vamos a una casa muy cerca del Café Mango. Cuando esta cerca a Mango Cafe (aproximente 2 millas al sur del transbordador, podrás ver Mango Cafe a su derecha) ... Ir directamente a una cuadra más y ir a izquierda en la calle Emiliano Zapata. Casa Roca Caribe es un bloque y es una casa blanca con dos plantas."
You should print that out and then just hand it to them when you get in the taxi.
The taxi ride is 50 pesos plus 20 pesos per bag...so it is very inexpensive...when you arrive, just pay him and don't allow him to charge you more than that. if he does, you were just charged a 'gringo tax'.