Looking for someone in Cozumel now with a G2 computer (with the charging cable).
I am here and forgot my cable, battery running low.
Ask on cozumel4you facebook page. There are massive amounts of divers on there that can help you out!
We went there a few weeks back. We were impressed with inventory and super friendly and knowledgeable staff. We didn’t see too much in regards to dive computers. He had a few there, but I bet he could order what you need.
This is why I like my Shearwater Perdix AI - it uses AA batteries (of any sort), which are available nearly anywhere, and it syncs to my laptop, phone, or tablet using Bluetooth. No cable to lose!
Check not only with the Cozumel4You Facebook group and Scubaboard's Cozumel forum, but also with your dive shop. As you can probably imagine, people lose stuff and leave things behind, so they'll often have a collections of bits that they know might be useful in the future. If you're staying at a resort, check with the desk. When people leave stuff behind, it's more likely to be at the hotel than on a dive boat. They usually keep anything that looks like a charging cable that might help a future guest.
Also check with ProDive. I'm not sure what computers they carry or if they'd have spare cables for sale (and if they do, they'll be expensive there). A couple of the other larger stores are possibilities, but you'll have to balance how desperately you need a cable against the hassle of getting around and checking them.
Absolute worst case, and possibly worth it if you're staying for long, is that you can have stuff shipped to you (using FedEx or DHL) at PakMail and pick it up. I would not have anything shipped to me even at my own house, but PakMail is reliable.