I would definitely take in everyone's input here, but an AT&T iPhone 4 or 4S has been wonderful for traveling in the Caribbean for me. When I'm traveling for pleasure, I often buy local SIM cards, and when traveling on business the AT&T SIM works great (at their expense).
It can be easily unlocked by anyone who is reasonably technically savvy, and A paper clip is all that is needed to swap out SIM cards. Having an iPhone also lets you have a great backup tool for
GPS/chart plotting.