SHARKS!!!!!

I figured I'd start with probably the most popular marine animal. Sharks are fish, but instead of bones inside of them, they are made out of cartilage. Humans have cartilage on the tips of our noses and the tops of our ears. Most people are afraid of sharks since movies (Jaws) portray them as viscous man-eating creatures. Most species however tend to avoid people. In the Caribbean there are a few different species of sharks that divers are more likely to see.

Nurse Shark

One of the most popular, and probably the coolest shark in my opinion is the nurse shark. Nurse sharks, unlike most other sharks, live on the bottom of the ocean. They don't need to swim in order for water to pass over their gills. Instead they can stay stationary and pump water through a device called a spiracle. Nurse sharks look a lot like catfish. But unlike catfish they can grow to over ten feet in length. Their bodies are a purple color and when they swim, which is not often, they have a yellow tint around them. Living on the bottom they tend to eat bottom dwellers. The barbels on the front of their face can detect creatures under the sand, which helps them find food. Unless they are bothered, they leave humans completely alone. However, if a human does something stupid like pull their tail, they'll bite the human and not let go. It's just better if you don't bug them.
  The Caribbean Reef Shark is another common shark found in the tropics. They look like what most humans classify as sharks. So long as they are in an open area  with plenty of ocean to swim in, they won't go near humans. They give birth to live young between November and December. Another shark like them are sand tiger sharks.  Since both species must keep swimming in order for water to pass over their gills, they have the ability to shut down half their brain at the time so that they can rest it.
Shark teeth are very common to find on a beach. Since their skeleton is made up of cartilage, there is no hard jaw for teeth to stick to. Instead teeth tend to fall out when a shark eats. To compensate for that, sharks have rows and rows of teeth in the back that move forward everything they loose one.
In short, if you see a shark while diving, that's really cool. Keep your distance and watch it swim, but don't try to get in the way.