The Great Deep

The Great Deep is a biome that spawns when a world is created. The ocean goes for miles now and goes very deep down. It is separated from the ocean by what appears to be a large doorway. It will ask the player for a Deep Sea Key, which will be dropped by any sea enemies with a 0.1% chance (0.2% in expert, 0.4% in master.) When the Deep Sea Key is used on the door, The Great Deep will be permenanetly unlocked for items and enemies. A lot of weapons are harvested from there too.

Enemies (Pre - Hard Mode)
Yellow Jellyfish

Nightmare Clam (If the world has the Corruption)

Bloodthirst Clam (If the world has the Crimson)

Squidlet

Spiky Blue Jellyfish (In Expert Mode Only)

Spiky Pink Jellyfish (In Expert Mode Only)

Spiky Yellow Jellyfish (In Expert Mode Only)

Green Jellyfish

Shark

Vampire Crab

Enemies (Hard Mode)
Brown Jellyfish

Void Jellyfish (Post moon-lord only)

Supermassive Clam

Super Shark

Colossal Squid

Vampire Squid

Giant Squid

Void Leviathan (Post-moon lord only)

Bloodfeaster Crab

Insanely Enraged Whale

Fishron

Enemies (War Mode)
Spiked Void Jellyfish (Expert mode only)

Void Shark

Void Mimic

Depth Mimic