The only advice I can give is to check when you get on the ship and see if they have a port consultant. They usually give good shopping advice. They always told us that things were cheaper in Cozumel than in Grand Cayman, but I don't know if that meant electronics too. This year we went to St. Thomas and they said it was better to buy electronics in St. Thomas, than in a foreign country due to the warranties. It would be better in a US territory. Grand Cayman might be pretty reputable but the price might be higher. The best thing like I said would be to ask the port consultant. They usually have all of the ins and outs of things and they do a lot of checking out these ports.
Correct me if i am wrong, but i think that neither of those ports are "duty-free" ports. No deals on electronics. All i saw in Caymen was touristy stuff - t-shirts and that kind of thing. Cozumel has lots of jewlery stores and you can get some deals on loose gemstones, You might be better off buying your digital camera at home.