Best advice, if you hadn't started diving yet is to spend the time and interview instructors. There are a couple of different agencies that certify open water divers...PADI and SSI being the big two and while there are some slight differences they aren't as big as some would have you believe. Either will get you qualified to blow bubbles locally or in an exotic local; dive boats, resorts, and rental companies will recognize either card equally. What are not the same are the instructors. Some courses are very formulated...show up for the next two weeks, stay awake, don't kill yourself or anyone else and we will issue you a card. Some are quick. I have a mate that got an open water card in three days. My initial instructor told me if I was worried about how long it would take to go to someone else to learn to dive. He would issue my card when he was comfortable that I understood the principles, could repeatedly demonstrate the skills, and was comfortable in open water. I probably burned twice as many cylinders of air as some and more than most, but that was what I was looking for in an instructor. I wanted to be sure I "got it". But that's me. Be honest about what you want from the course, then look for an instructor that will provide that experience. Remember, it is quite literally, your life.