Sharks will have the ability to see movement on the water surface and will go after anything that moves. They have high attack power (2 on easy, 3 on normal, and 4.5 on hard) and a sturdy about of defense, so attempting to fight a shark with bare hands and no armor can be fatal. They are found commonly in pairs and in groups of 4. If you attack one, they will all attack you.
There is a reason why sharks have so much defense: low health. Striking one with an Iron sword will kill it in one hit. One thing also, they are very slow and can easily be outswimmed.
Sharks are found in Y=65 or less and can be seen attacking dolphins. If you are able to get close enough, they will make a growl-like sound.