Include a swimsuit, one change of clothes, basic toiletries and essential dive gear such as your mask, computer and regulator. This way, if your luggage does get lost, you're still ready to dive. Carry all your important documentation and valuables in your carry-on luggage or on your person. Carry a list of emergency phone numbers and addresses. Be sure you have all your dive gear and that it is in good condition.
Diving is a strenuous sport.
Be sure you are in good cardiovascular condition. Start conditioning months before your trip, not days. Check out Belize on the Internet to learn a little about customs, traditions and non-diving activities.