Boy, have you walked into a hornet's nest on this one. Let's start with the physical hazards you'll face while you're teaching new SCUBA students to dive. Realistically, your classroom work is pretty innocuous, except you'll probably lose your voice from telling students to stop texting while you're teaching some important dive concept. Once you get into the water, though, it's a whole different story.
First, the students have to learn how their dive equipment works, which means they have to put it on. Masks and snorkels are pretty much a no-brainer, although all bets are off when it comes to tanks and regulators. You might easily get a black eye from a student whose tank gets away from her while she's putting on her buoyancy jacket (with the tank connected, of course). You could nearly get garrotted from an out-of-control regulator that wrapped itself around your neck during the same operation. Even worse, you could get bulldozed into the water when a fin-wearing student crashed into you while weaving across the pool deck. Let's assume, however, that you survive all of these mishaps. Now it's time to get in the water.
For discussion's sake, let's assume you're in the ocean, not in the pool. Now you've got hazards coming at you from every direction. Marine life can sting you, bite you, or give you a nasty case of swimmer's itch. Diving can also cause complications for a wide range of medical conditions. The most well-known set of risks, however, pertains to incorrectly interpreting the ironclad dive time/depth rules (or ignoring them), and setting yourself up for ailments ranging from ear squeeze to potentially fatal decompression sickness. No need to go into the details; just understand that SCUBA diving is not a sport where you can bend the rules to suit your whims. Enough said.