Another vote for the ferry, we have taken the Tank-Ha boat over twice & while Tank-Ha was (is still?) a great shop the crossing was very rough, especially coming back. We have also done the ferry & found it much more relaxing, we can recommend Deep Exposure as a great operator out of Cozumel to go with. They have a larger, more comfortable, enclosed boat than most shops, they will pick you up at the dive boat pier right beside the ferry terminal, they provide a great lunch & will take you to sites away from the crowd. They even added a second DM to the boat at no extra charge after talking to us & realizing that they had 6 divers, 3 of which were very experienced & 3 of which were newbies. That way the experienced divers could dive with one DM & the newbies with the other & everyone had a great time. Definitely the best operator that we have been out with over several trips to Cozumel.