Ocean

The Ocean is a large Water body found on the surface of each map, at its far left and right edges, comprising the eastern 500ft (250 blocks) and the western 500ft of the map (at least on Large maps; it is closer to 150 on Small). It extends as deep as the Cavern Layer, with its maximum depth reached at the very edges of the map.

Oceans will commonly contain chests with loot pertaining to water. They also contain unique enemies, like Crabs and Sharks.

Farming
If you have the Vilethorn, you can easily farm the Ocean for Pink Jellyfish (which drop large amounts of coins) and Sharks (for Sharkfin, or to find the Diving Helmet). To do this, take a supply of about 50 Blocks with you (any type except Sand) and sink to the bottom. Quickly seal yourself into a cage of blocks with at least two tiles of head room height above you. Fill in the head room with blocks and then mine them away with a Pickaxe (or remove water with Buckets). You have now created an air pocket where you can breathe, and you'll even have natural light during the day, as the sky will show through your air pocket. [Note: This no longer works as of update 1.1, since liquids now occupy a background layer and are not "replaced" by blocks!] Instead, once you have sealed yourself in on the sea floor with your choice of block, dig into the sand below, thus causing the water to drain into the newly made hole. Then (if you wish), once you have a decent air pocket, you can place blocks over the water that drained into the sand blocks you just made. That way, you don't have to constantly jump. When you're ready to collect all the drops, you will probably need to extend your cage towards them. Place blocks outside your cage to thicken the left or right wall (whichever direction you need to travel) and then mine away the inner blocks. Repeat until you reach your destination.

Better yet, make a grinder. Put Wood Platforms in the top tile of water, then put a thin layer of lava over that (at least six tiles wide, but narrow enough you can grab all the drops automatically). Then build a cup of water to stand in, not connected to the ocean, directly over the center of the lava. Wield water candle and drink Battle Potions if desired, and leave the game running unpaused while you do something else.

Other strategies
Unique items attainable in the Ocean include the Shark Fin and Diving Helmet (dropped by Sharks), and Coral (grows on the Ocean floor). There are also chests, though they rarely contain items that aren't also available elsewhere.
 * If players without proper accessories try to swim to the side of the map, there is a serious risk of drowning. In addition to plenty of healing potions, it is advisable to bring items and potions that aid in breathing, such as the Breathing Reed and Gills Potion, a Magic Mirror for returning home quickly, and Flippers to aid swimming. The Grappling Hook or Ivy Whip may also be of some help in returning to the surface for breath.
 * Players may attempt to drain the ocean by tunneling down to the Underworld, where water evaporates. The Ocean is in fact drainable this way, and this method will allow the player to go back to the Ocean floor and easily pick up all the Coral without worry of suffocating. However, no Sharks will spawn once the Ocean is drained, and previously-spawned Sharks will have vanished. Therefore, draining will not allow a player to obtain Shark Fins and the Diving Helmet.
 * For looting Ocean chests, generate a small or medium-sized map. Then, using the Shadow armor, Hermes Boots, Anklet of the Wind and other speed boost items, run from one side of the map to the other. Since there is a good chance to have some Chests in the ocean, it is an easy way to get medium-tier items. The chests will usually contain items such as Breathing Reeds, Shurikens, Magic Mirrors, Dynamite and Coins. When doing this, a Magic Mirror is also helpful, so that you don't need to run all the way back.