Dunebuggy and snorkel in Cozumel?

My wife and I are looking into the excursions at Cozumel, Grand Caymen Islands, and ocho Rios... We first said that we wouldn't book it until we got off the ship cuz people say it's cheaper, but I do know some of them sell out early. Anyone done the dunebuggy and snorkel before, and what's the best port for excursions?