If you are really meaning Cozumel (because I saw you asking earlier about Costa Maya) what type of excursion are you looking for?
Do you actually want a tour to somewhere, or are you asking if you can get a taxi to a beach or something? What type of activity are you actually looking for? Do you just want to go somewhere to snorkel from shore (no beach) or a boat snorkel? Again, which pier are you docking at - it can make a difference to budget.