Really? You are going to such pretty exotic locations and you want to go fabric shopping? I have been to two of these ports but I wasn't looking for fabric. Why don't you tour things you won't normally see? Costa Maya had some great tours. The port is very touristy with loads of gift shops, pharmacies (for low cost meds), bars, jewelery shops. My DH and I went snorkling instead at a coral reef and there was swimming with the dolphins at the same location but very spendy. There was also a gorgeous beach to just relax and have a cocktail or two. Cozumel I would book a tour to the Mayan ruins. Take a sketch pad and view the hieroglyphs for quilting motif ideas.
Batiks are made in Indonesia so if you want to go on a Batik fabric buying trip you need to go there.