Introduction
Let's face it, oceans are perhaps the most boring thing about Minecraft. It's just miles and miles of water! Is there anything for you explore? Nope, just water. Are there exciting new mobs? Nope just water. I mean seriously, have you ever thought to yourself "Let's go to an ocean! It'll be so much fun!". Chances are you haven't, and for good reason too! Oceans in their current states are just big, barren biomes that provide no challenge, nothing for you to explore and few resources. But there are a few things I have that may turn these wastes of chunks into a biome full of life and useful components!

Sunken Ships
These will naturally spawn rarely in ocean biomes under the y axis 70. The ships would function similarly to dungeons as they will provide a loot chest if reached successfully. These would no require a mob spawner to "guard" them as I feel swimming all the way down to such depths is enough of a challenge as it is. As for appearance, the sunken ships would appear to be heavily damaged sail boats, usually found half buried into the ground.

New Mobs
First of all, let's start with an obvious choice. Fish. After all we already have them as items, so why not as mobs. Much like horses, they would spawn in a wide variety of colours and patterns. They would also be relatively small, being on average no bigger than a block and only posses half a heart of health. On death, they would have a 75% chance of dropping raw fish. These would bring the oceans to life and make them more aesthetically pleasing.
Let's also take a look at a few potential hostile mobs! This would make sea travel a fair bit more challenging than simply hopping in a boat without a care in the world. An obvious choice would be the ever so popular sharks. Much like the fish, they could spawn in a variety of different patterns representing various species. They would posses a whooping 15 hearts of health! This would create a greater challenge for the player. However they are heavily rewarded for slaying these mighty beasts! Perhaps sharks could drop shark teeth (used for crafting various tools and weapons) shark hides (used for armour) and perhaps even raw fish (best part).
How about jellyfish? They would bob around the surface of the oceans, presenting no real harm... until provoked. They would be two possible ways of provoking a jellyfish; by attacking one or hitting it with a boat. This would create a much greater challenge for navigating oceans by boat! These jellyfish would prove to be ferocious in combat. While only possessing 7 hearts of health, they could dish out a crippling poison effect onto their attackers. Once slain, jellyfish would drop raw jellyfish. These could be used as an alternative ingredient for brewing poison potions and used to tame sea turtles.
Finally let's end the mobs with my personal favourite, a sea turtle. These could be tameable mobs that are befriended upon being fed 3-7 raw jellyfish. They would be ridable, much like horses, but painfully flow on land. To make up for this handicap, they would quick by water. They would also provide as a defence against jellyfish as rather than provoking them on contact, the turtles would simply devour it!

Decoration
Beautiful aesthetics such as coral and seaweed would be scattered on the ocean floor. Seaweed would serve as an edible crop that could only be grown underwater. Coral could be harvested through the use of shears and brought to the surface. The would spawn in a vibrant display of colours, without a doubt brightening up the barren ocean floors. Perhaps seashells could also be found, serving no purpose other than decorative tiles to pretty the place up.

Currently there isn't much to my idea, but there is so much more to be expanded upon! Atlantis like villages, Krakkens or even enemy pirates! The possibilities are endless. With a different point of view, we can turn a barren deposit of a biome into a blank canvas full of potential! Perhaps this is all just opinionated on my part. So let me know! How do you think this idea would hold up? Would you like to see this (add) or despise it entirely? (not to add).