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. Bail out the room you've created with a Bucket (or several). 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. Alternately, 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.

Ocean enemies seem to require three block spaces of height in order to enter an area, so it may be possible to use the cage method with other weapons that don't travel through walls, by leaving a space for another ranged weapon to penetrate.

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.
 * Another way of draining the ocean is by making an air pocket, empty an bucket of lava in it and place an object that can't be destructed by lava in the lava. After that allow the water to flow into the lava. The object will prevent obsidian from being generated, causing all the water to dissapear.
 * 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. This way you can easily obtain the Breathing Reed, Trident and Flipper.