Even if contrary to scuba diving, snorkeling isn't considered a dangerous recreational activity and doesn't require any certification at all, the same as in any water sport, some hazards and dangers do occur, which a beginner snorkeler should be aware of.
Most snorkeling associated injuries are caused by reflex actions leading to harmful contact with otherwise harmless objects like reefs, stones, boats etc. rather often resulting in some nasty things like, say, scraped elbow or knee. Mind that under water, especially in tropics, even small scrapes tend to be easily infected.
Dangerous marine creatures?
Sometimes the same injuries are due to simple lack of knowledge. The underwater life and marine aquatic creatures are very colorful and arouse one's natural curiosity, but better watch and don't get tempted to touch anything you are not sure what it is.
Not speaking of the extremely rare instances of snorkellers provoking some sea creatures to attack them (say, accidents were reported, when wearing jewelry or something metallic provoked barracudas to attack snorkellers on Cuba's resort outside of Holguin), normally the injuries are due to diver's voluntary contact with the creatures that can be only dangerous when touched or stepped on, such as jellyfish or black sea urchin. Learn, what to do, though, if there's an unprovoked shark attack.
Getting over chilled is another danger. The point is that in the water the human body gets quickly accustomed even to the temperatures rather lower that can normally be afforded, and you can start freezing before you realize it, which in the worst case can result in muscle spasms. Once you have noticed the very first signs of feeling cold, get out of the water immediately, and don't resume snorkeling until you are completely warmed up.
Staying warm is crucial. If you feel cold in the water permanently, or if you see you get chilled too fast, then you need a dive skin, a wet vest, or a lightweight wet suit, depending on how much thermal protection you need to feel comfortable in the aquatic environment.
Not to be overlooked, sea currents can be very dangerous, even the smallest ones can be misleading. In no time you can find yourself being caught into a stronger one and swept out to open sea. The main rule is not to try swim directly against the flow back to shore. Swim to the side or slantwise out of the current, and don't loose control, don't give the way to fear!
Remember, contrary to many other water sports, in snorkeling the true measure and the real indicator of your skills is your ability to stay relaxed. Speaking in the terms of confronting a danger while snorkeling, the best response to a dangerous or potentially fatal situation is to stay calm.