Relax, watch your bouyancy, relax, don't fight the currents, relax. Seriously, a light current is the absolute best diving in the world. You exert less, which means lower gas consumption. If you want to stop to get a better look at something, you can back kick or (if you've not mastered that skill yet) turn into the current and fin to hold your position.
The boat will follow your bubbles. A DSMB will be shot before the assent, so the boats know exactly where you are.
Pay attention to your group - getting seperated is a bad idea.
Occassionally two groups will pass each other - I've never been sure why the faster moving ones don't SLOW DOWN, but whatever. If this happens, and the faster moving group passes through your group, keep an eye on your group. I've seen a person from our group start to speed up and follow the other group...
There are lots of swimthroughs off Cozumel, especially on the deeper dives. Strict bouyancy and frog kicking so you don't silt it up for the people behind you. If you're not comfortable in swim throughs, make sure the guide knows this ahead of time.
Carry your own SMB and other signaling equipment, just in case.
If the current is too heavy for your comfort, thumb the dive.