Minecraft wiki entry for Best_biomes_for_homes, associated text: All variants except for warm oceans contain kelp, which can be used as an underwater "crop" for food.
While warm oceans lack kelp, they are the only variant to generate coral reefs and sea pickles, which are a spectacle.
What variant you're in depends on what type of underwater mobs spawn. Warmer variants spawn tropical fish which come in thousands of color combinations, as well as dolphins and pufferfish; Cold (but not frozen variants) will also spawn dolphins as well as cod and salmon; squids spawn in normal and frozen variants.
Dolphins, when fed fish, will lead the player to a nearby underwater structure chest, likely a shipwreck or underwater ruin. They also provide the player with a Dolphin's Grace effect‌[Java Edition only] or instant swimming boost‌[Bedrock Edition only] that increases their swimming speed when swimming alongside them.
Turtles actually only spawn on beaches, but you will generally find them swimming around in the adjacent ocean. They will often head back to their home beach to lay eggs, which can be collected with silk touch or observed until they hatch to harvest scute from baby turtles.
Both underwater ruins and shipwrecks can contain valuable loot such as crops, books, paper, iron bars and nuggets, gold, and even diamonds and emeralds. However, their most valuable loot is the buried treasure map, which leads to a buried chest in a nearby beach biome. These chests contain more valuable minerals, as well as one heart of the sea, which is the main component to the conduit.
Drowned, while difficult, can sometimes drop nautilus shells (used for making conduits), or tridents, a weapon that can be used as a sword or thrown. It can also be enhanced with unique enchantments like Channeling, which calls down a lighting strike; riptide, which pulls the player along with the trident as it is thrown; or loyalty, which causes the trident to return to the player once thrown.
Abundant supply of ink sacs.
Ocean monuments are found in deep oceans. This is the only source of resources like sponges, also one of place where sea lanterns and prismarine can be found, and can be used to create guardian farms.
A good, deep source of water for fishing in.
Fish can be caught with water buckets, allowing for the player to move them to their desired location.
In factions or anarchy servers, this is the best biome so set up a base. You can build an impenetrable base under the bottom of the ocean or in the monument while nobody knows.