Offroadr, PADI isn't the only valid certificate. It really depends on the instructors and course content. Our local scuba shop offers PADI certification with only the minimum required pool time to certify. The technical institute I graduated from offers a ACUC sport diver certification. The ACUC course was worked into the school year, so we spent 3 months snorkeling twice a week, and 3 months scuba diving twice a week. We likely had in excess of 100 hours of pool time, 50 of that in full scuba gear.
Four years later, in Jamaica, I decided to join a reef dive. I grabbed my rented gear, checked all components, and had it on and functioning in a matter of 20 minutes. The "resort" divers in the group were still fiddling with their equipment, trying to figure out what goes where.
When we finally got to the dive site, the partner I was paired with was so nervous he was gulping huge volumes of air. He was in the water for less than 30 minutes, at 30 feet, when he ran low on air (500 psi minimum enforced by dive master). I spent an additional 40 minutes with one of the dive masters before I ran out of air. The reason is simple....comfort and confidence gained through an excellent training course.
You simply cannot absorb enough information in a short resort course to be confident in the water. If you are confident after a short resort course, you are a danger to others in the water. The emergency procedures are not second nature to you, you have poor buoyancy control, and you don't recognize dangerous situations as they arise.
A wreck dive for an uncertified diver (or resort cert) is a VERY bad idea. Leave Wreck diving and Cave diving for those who are certified. The life you save may be your own, as well as those that have to recover the body.
That being said, SCUBA diving is an unbelievable adventure. I would highly recommend diving while you are on your holiday, but I would also recommend you get certified by a reputable organization well before you go on vacation. You will enjoy your diving vacation much more if you are relaxed in the water. And remember, leave it the way you found it. Don't touch the coral and don't take souvenirs off the bottom.
Happy Diving...
Neil; ACUC Sport Diver since 1988.