The R1 and R2 city buses run continuously along Boulevard Kukulcán between the Hotel Zone and downtown, day and night, for around 15 pesos. They are reliable, frequent, and far cheaper than taxis (which run on agreed-not-metered fares that triple after midnight). Inside the Hotel Zone, walking between hotels is usually painful — the boulevard is long and there are no proper pavements in many stretches.
For Isla Mujeres, ferries leave Puerto Juárez (downtown side) every 30 minutes; the more expensive Ultramar ferries leave the Hotel Zone marinas. For Cozumel and the Riviera Maya, take ADO buses from the downtown terminal rather than the much pricier hotel-arranged transfers.