I have taken the ferry a few times, once from a cruise and other times when we stayed in Cozumel for a week and found it to be an effective and safe way to see Playa Del Carmen. I had also had an excursion that used the ferry to get us to the Mayan Ruins of Tulum, which if you find history interesting was great
This excursion also included the bus from the ferry to the ruins. I have also stayed in the Playa del Carmen area a few times and found it to
a really nice area and the beaches were incredibly beautiful. You won't be disappointed!