ok well ive been on 2 criuses that went to cozumel. they dock you in like a pretty secluded place, surrounded by a bunch of shops (none of them pharmacies). your going to have to know exactly where to go because anywhere you need to go is going to be by cab. cozumel is a pretty nasty place honestly, and i wouldnt even fuck around going into the city.
oh, and coming back onto the boat is a JOKE. i went and bought a fifth of vodka from a duty free store for like 5$. and crotched it on the way back on. they run a metal detector over you, so i wouldnt worry. if you have a bag with you...dont put it in the bag, they run those through an x ray machine. make sure it isnt visible in your pants though.
i personally wouldnt bring anything like that back into the country because of customs. leaving the USA is a totally different story.