The snorkel seems simple-basically, it's a tube that you use to draw air from the surface when your face is in the water.
However, some variables may be more than you may appear initially. The size of the hole (tube) of the snorkel is important. It must be large enough to take a deep breath quickly and easily, but not so large that it unnecessarily increases the amount of air that must be moved back and forth with each breath.
The length of the breathing tube is also important. It must be long enough to stay above the water when swimming, floating or otherwise resting or working on the ground. But snorkeling for too long can be awkward, and in fact it's harder to breathe.
Gear manufacturers are very familiar with all these factors, as well as the ergonomic design that makes breathing tube use comfortable. So if you choose a reputable brand from a professional diving retailer, you can rest assured that you will get a reliable diving gear.