Deep sea biome

It would be really interesting to add a biome at the bottom of the ocean.

The idea would be to have hostile mobs that could, for example, take us to the bottom of the ocean.

We could find a new mineral that would allow us to breathe underwater, for example.
We could also find a "Sea Mining" enchantment that would enable us to mine faster underwater.
Another called "Water speed", which would enable us to move faster on the ocean floor.

New structures, such as underwater villages with villagers offering us fishing-related trades.
The remains of several types of boat, perhaps with a new type of treasure? 
Vestiges of abandoned cities, large "Atlantis"-type structures perhaps?

Many monsters would be interesting in this update, such as lanternfish, which could be caught in a bucket and used as a light to move around. 
Slugfish, viperfish and other types of fish are also of interest.

And finally, I think we could have a boss, a kraken, it's a cliché but it would look really good.
And combat could be great, but you'd need armor and the enchantments to go with it.