Is it necessary to rent a car to go to the city centre?
No, you can take a taxi or public transport. There is a taxi stop in the hotel.
Is there a public bus service? How often do they run?
There is a public bus service. You will find a bus stop in front of the Resort and for a few pesos you will be able to go from Tulum to Playa del Carmen. The bus runs every 15 minutes until 7:00 p.m. (This service does not belong to the hotel)