Devil Fruit users in the future will be unable to use DF moves within the water and will also lose stamina upon entering the water. It is unknown if DF immunities will be affected when in water though.
Shipwrecks are a new addition as of 2.0. They are decent sources of gold and are the first major objects made in the ocean. An air bubble effect happens on the rocks around the shipwreck in-game.
Fishmen are capable of breathing underwater and also move faster when underwater, as well as get increased combat power (only when in water).
A glitch sometimes occurs where you sink below the ocean floor to your death.
Diving was added in v1.9
As of v2.0 devil fruit users can still swim into the ocean, but move extremely slow. Devil Fruit powers, though strangely enough, still work underwater. (This will be changed in the next update)
All enemy mobs fall through the ocean floor at the moment and it is unknown if this is intended or not.
If Party Table Kick Course is used while on the ocean floor you will be able to move about while in mid-spin. This is very helpful for undersea combat.