I have an XM Airware and I am also going to the Caribben in Nov. I thought that you had to physically be in the US in order to get reception? At least that's what I have read on other boards. Is it different for Sirius?
The XM and Sirius sats cover a good portion of Mexico, the Caribbean and Canada. The signal is somewhat weaker than in the continental US, but you can get a usable signal. If you have your antenna outside and oriented properly, you should be OK.