Ocean

Both edges of every world end in ocean: the outermost 338 tiles on each side, a sloping sand beach that drops into deep water running out to the world border. Palm trees along the shore cut down for Palm Wood, Coral on the seabed, Seashells and Starfish on the sand and in the shallows. Those last three regrow over time, so a beach you have stripped bare will restock if you leave it alone and come back.

Sharks, Crabs and Pink Jellyfish are the regulars, with Squids and Sea Snails turning up more rarely for Black Ink and Purple Mucus. None of them will kill you as reliably as drowning will.

There is a useful quirk in how they hunt. Ocean enemies using the swimming and jellyfish behaviors only notice you once you are at least a tile deep in water, so wading in the shallows keeps them from committing and lets you fight them one at a time at the edge. Go deep and they all come.

Bring breath first and healing second. A Breathing Reed or a Gills Potion buys you time, Flippers fix your movement, and a hook is often the fastest way back to air when a swim goes wrong. Keep a Magic Mirror on you to bail out entirely. The permanent answer is Neptune's Shell or anything built from it, which removes breathing from the equation and makes the whole biome trivial to work in.

Water Chests sit along the seabed and carry water-themed loot. Whichever ocean shares a side with the dungeon also has a one in three chance of generating a cave: a flooded tunnel roofed with hardened sand, starting at the ocean floor and running inward, decorated with long strands of seaweed. That tunnel has an extra chance of its own water chests, and those roll each of the primary items with equal weight, which makes them a better place to chase one specific item than the surface chests are. Drunk, Celebration Mk 10 and Zenith worlds always generate the cave.

One world in four makes one of its two oceans Florida style, which flips the slope of the beach so the long shallow stretch runs inland and the ground drops off sharply near the world edge. A Florida ocean can still roll a cave.

Separately, the seabed sometimes generates below the surface layer. When that happens you get underground background showing behind the water and seaweed growing on the sand, and if it sits low enough, campsites, desert underground cabins and the occasional Life Crystal end up on the ocean floor. Worth a dive on any world where the water reads deeper than usual.

Ocean fishing checks the bobber, not you, so you can stand on dry beach and still pull ocean catches as long as the bobber lands in ocean water above the surface depth line. The catch is purity: any corruption, crimson or hallow in range overrides the ocean pool with its own. There is one odd interaction worth knowing, since the ocean counts as a desert internally. Ocean catches normally beat oasis catches on priority. If the ocean is hallowed, hallow catches jump above ocean, and because a hallowed ocean is also a hallowed desert you end up pulling oasis catches about half the time.

You cannot create an ocean and you cannot expand one, but you can widen the water inland as far as the natural beach reaches. What the ocean does give you is space. It is the flattest, widest open ground in the world, which makes it the standard choice for large arenas and farms that need room.

If you want the water gone, the reliable method is a shaft from the ocean floor down to the Underworld, where water evaporates on arrival. The other trick is to open an air pocket, empty a Lava Bucket into it, then place a piece of furniture in the lava. Water flowing in vanishes instead of making obsidian, because the background object prevents the obsidian from forming.

Protecting an ocean from hardmode spread catches people out. Digging a horizontal gap under the seabed does not work, because world data continues a little past the visible edge of the map and the spread crosses beneath your trench. The gap has to be vertical, cut down through the beach itself.