You can do the majority of the studying and course work online prior for the Open Water certification on the PADI website, then once in Belize, or even a local PADI dive course, take the classroom portion, though much if it is under water, so its always nicer to learn in a beautiful marine atmosphere than in a swimming pool or lake. Its also more realistic, as the majority of training is safety, safety devices, and safety procedures in the event of a problem underwater with you, or your buddy when diving.
Resort Diving is fine, but you will be limited to about 25ft depth, vs 50-60ft, and also will be limited depending on other conditions like currents, water clarity, and how controlled the dive area is for you and the divemasters to ensure your safety.
I highly recommend getting your PADI OW certification, not only will be able to enjoy many wonderful places to dive in the world, it is also another great way to learn and understand marine biology, marine protection and also marine bio diversity. If you dont remember everything after a few years, you can go online to refresh yourself, or even take a quick refresher PADI course when you arrive at the dive destination to review all the safety drills/procedures.
The diving community is a lot of fun, all sort of people from all walks of life, all ages, from around the globe, always a nice time, and everyone looks out for everyone when diving.
Enjoy!