In Cozumel you will have to pretty much read the whole thing to ID all the fish!
One way would be to just learn to ID some of the most common groups, and then learn the differences between the species.
1. Angelfish. Queen, French, Blue, Gray
2. Parrotfish. Rainbow, stoplight, and queen. Blue and midnight.
3. Filefish. whitespotted, scrawled.
4. Cowfish. honeycomb and scrawled.
5. spotted drumfish, particularly the juvenile. Highhat.
6. Grunts. Bluestripe and french (and then you can help ID when a new diver on the boat asks the DM about them and the DM can't figure out what he's asking about. So common that the DM "doesn't see 'em".)
7. Horse-eye jack.
8. Barracuda.
9. nurse shark
10. Cleaning goby, neon gobies.
11. yellowhead jawfish, splendid toadfish, garden eels (not related, but all things that the DM may stop the group to observe, along with eels.)
12. Seargant Major and their egg cluster.
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For shore diving, you will be looking at lots of different damselfish, blennies, and gobies.