Basic Open Water certification can be done in 3 days in most places (standard class is a week, but they have 3 day accelerated courses), and on vacation is how most people end up getting certified. But keep in mind it's 3 FULL days, so it's not something to do if you're wanting to spend your nights out doing other things (very draining).
There's also a lot of places that will take you out one time without a certification. You usually go in for about 3 or 4 hours before the dive and learn the very basics that you'll need for an easy dive, and when you go out the instructor is with you the whole time. Overall a lot cheaper, one day, and you get a taste of it before getting into the full thing.
BTW, for this post and the one above, I'm talking PADI certification (which I recommend, look them up). There are different certs that I know nothing about.