We were last week in Cozumel with the freedom and went to Tulum by our self.
You have to look for the exact ours of the ferry. It was 10 $ pp for the ferry one way. The taxi for 7 persons was 20 $ pp. we took the ferry of 7am local time (ships time 8 am) It took about 50 min to get to Tulum. And we had the return ferry of 11 am. There is no ferry between 11 am and 1 pm. We had still time to do some shopping in Cozumel before returning to the ship. If you take the ferry of 1 pm, you need to go strait to the ship. our ship left at 2.30 pm local time.
Tulum is a very nice Maya temple right on the beach.