Your snorkel is more than just a tube to help you breathe air with your face in the water. Well, okay, that's exactly what it is, but there are still options to consider when purchasing a snorkel. You want to find a snorkel that helps you breathe easy in the water, while keeping the water out. Maybe you want a semi-dry or dry snorkel with vents or valves to keep water out of the top? Maybe you want a purge snorkel with a one-way valve at the bottom to let water flow out easier? If you think you may want to move on to scuba diving at some point, a snorkel with a flexible lower section that allows the mouthpiece to drop out of the way when you aren't using it is a great idea.
Most importantly, when choosing a snorkel, you will want to find a comfortable mouthpiece. Jaw fatigue can cut give you a headache which will cut a day of underwater exploration short. You need a snorkel that fits your mouth comfortably, while breathing easily.