You have the time to train and certify here in California before your trip, so there is no reason to wait. I have this conversation with many of my new students. All the points above are valid. There are some other things to consider. No everyone certifies on the first attempt, for a variety of reasons. Some new students need some additional time to master a particular skill, or need to give their ears a rest before getting back in the water, or whatever. Take the time you need here at home. One of the added bonuses is having time to dive here and become better/safer divers. It will make your time in Cozumel much more enjoyable. Lastly, if you learn here, in our cooler water with more thermal protection (and weight to sink it), it makes diving in the warm, clear water of Coz seem SO easy!