Actually, I always recommend that, if at all possible, before your trip, complete your open water certification at home. The reason that I recommend this is because the conditions at home are probably much harsher than at your vacation destination. Therefore, you will have experience with more adverse dive conditions before you ever reach your holiday destination. Also, it is quite a bit less expensive. Then if you are still feeling a tad nervous when you arrive, take a special orientation dive with a dedicated DM. Handling your certification and introduction to the holiday diving in this manner, will save you significant dollars and help make you much more comfortable in the holiday waters, thus ensuring more enjoyable diving experiences.