1. Is it in the budget? The full open water course costs around $300. That's a big splurge for a backpacker, but totally worthwhile. Roatan is one of the cheapest places to get certified, and your certification is good for life, though you should get a refresher whenever you haven't been in for a dive in over 12 months.
2. Do you have the time? The PADI open water course takes 3-4 days, and you need to wait at least 12 hours if flying after diving (longer if you're doing repetitive dives, your instructor will review the current recommendations with you). If you're not staying long enough to do the course, you can still try diving with the Discover Scuba experience, which takes a 1/2 day, or the Scuba Diver course, which takes 2 days and results in a certification with restrictions.
3. Are you fit to dive? Before starting your course, you'll be completing a medical questionnaire. There are some medical conditions that aren't compatible with diving, or may need a physician's clearance before you can continue.
If you've got the money, the time, and don't have any medical factors to rule it out, what are you waiting for? Diving can be a life-changing experience. My older son got his junior open water certification when he turned 10, and his younger brother will be following suit soon, experiencing a whole new world underwater.