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I think you'd be better with a duckbill seal rather than open bottom
6' is good if you dive in seas with bigger waves, if not something smaller will do
I like the Buddy SMBs myself, and stow them in the pocket on my backplate where they fit well
I also have a small Halcyon oral inflate that clips onto a d-ring
Puffer Fish gave some really good advice - the bigger the SMB, the harder to deploy. Even a 5' SMB is going to have 30+ pounds of lift, so if you're deploying underwater you need to have your skills down
So my suggestion would be to get the smallest volume that will be suitable for the conditions. I have no idea what they are in Cozumel, no doubt someone will chime in