Are elasmobranchs dangerous to humans?

It depends on the elasmobranch.  Not all sharks are dangerous, but some species of sharks have been known to attack humans.  The largest shark, the whale shark, is actually completely harmless and humans can safely swim around them.  The most dangerous sharks are the great white and the tiger shark.  Rays pose a threat to humans due to the stinging spines on their tails.  Skates are harmless to humans as they have no stinging spines.