I am a scuba instructor and also own a scuba training/retail centre. I love diving, and am passionate about introducing it to others. But your plan (and your attitude towards it) worries me. I am not being critical of you, I just expect you do not really understand the process and why it is very important to be well prepared and trained before you even hit the open water.
There are certainly places that will “rush” you to be certified but I DO NOT RECOMMEND.
There is classwork/knowledge development and confined water/pool training dives you can do before you go on your trip (and get a referral for the open water certifications) but you need to do a lot of work in pool and be well prepared before the checkout dives (for PADI, there are 4 open water checkout dives. We spend 6-9 hours in pool with students in small ratios (1:1-1:2 usually) before even sending them into open water, which takes place over 2+ days depending. Many places will do the class/pool training faster and in larger groups (for cheaper) but I will say that you may not feel as prepared for open water or enjoy it. We want to turn out safe, responsible, aware divers who will also know how to respect and protect the places they dive, not people who may or may not squeak past their open water skills to be certified and may be a risk to themselves, their buddies, other divers, and coral and marine life.
Do not rush your training. Diving is fun, and I encourage people to get certified, but it also requires you to be well prepared and to be a safe diver so it stays fun (for you and where you are diving). That starts with making decisions now not to rush the training, and to choose an operation who also won’t rush you through it even if it costs more. Why would you cheap out on training and instructors that are trying to keep your safe? Also, compare apples to apples – cheaper costs often are not including materials costs, etc. I would at least recommend you do all the online and pool training before you go, with a reputable operation, and get a referral for the open water certifications from them.
j_jaye : No, you don’t fly right after diving because you are going to a higher altitude meaning a rapid ascent to lower atmospheric pressure, not because flying dehydrates you (and it is the ascent itself to a high altitude, not whether it is long or short haul, that is the issue).
You do not fly right after diving for the same reason you ascend in water at a safe rate of speed, to allow nitrogen bubbles to release more slowly. Nitrogen builds in your body while diving as you are breathing air under higher atmospheric pressure. You ascend slowly to prevent rapid expansion and allow some release, and you also do surface intervals to off gas between dives. You also give time to off gas before flying to reduce and/or eliminate those nitrogen bubbles in your body. If you fly right away after you risk all those bubbles expanding rapidly which can cause serious injury and death. Think of removing a cap of a pop bottle after shaking it. PADI standards are wait to fly 12 hours after a single recreational dive, 18 hours after multiple/multi day recreational diving.
While dehydration may be a small risk factor in increasing *susceptibility* to DCS, it may not be as big of a risk as sometimes stated (see research of DAN and Dr. Neil Pollock). The biggest risks for DCS are violating no decompression limits and ascending too quickly (unsafe diving practices).