Try Cozumel Coffee House or something like that on Coz. Its right off the main square and I think it has decent coffee. I'm no expert like you guys but its ok. The peanut butter coffee is good for about one sip.Jennifer, I agree with you about coffee on Aggressor, but I DO LOVE Aggressor liveaboards otherwise!
To be fair, I don't find coffee any good on any of my dive trips. Usually in one of those big carafe things that has been sitting for hours.
The owner is very passionate about coffee and really knows his craft.