I love scuba diving, and I highly recommend anyone who lives near the water or likes to vacation near the water to take lessons and get certified. However, be careful in taking advice from your scuba instructor on what to buy.
Beware of the "I think I'm going to buy that myself" scam. As you casually browse in a dive shop, some scuba instructor (often the one who taught you) will point out a piece of equipment to another instructor, completely ignoring you. The instructor will talk about how great it is and how he wants one. Often he is saving up to buy one or he has been waiting for the device to hit the shelves and now that it is here he is are going to sell his old device to buy the new one. My first scuba instructor tricked me into buying a dive bag by that method.
A related trick is that as you ask a shop employee about some item, a dive instructor will come over and say something to the shop employee, hardly even noticing you. He will say something like, "Have those finally come in? I'm going to buy one tomorrow." I don't know how the incentives are structured, but these events happen too often to be coincidence.
The scuba world is structured a bit like a pyramid scheme. When you first start diving, the equipment is very expensive, but dive instructors get huge discounts. This provides an incentive for divers to keep taking lessons so that they can eventually become an instructor and not only get the discounts, but make some money as well. Of course, they only make money if they talk other people into taking more advanced lessons, which gets other people more into scuba, who eventually decide that they want to be instructors too.