Say you are sailing on the seas and you encounter a whirlpool. You can sail into it but you will lose your ship. You enter a type of gravity level as you will be swimming up and down. You have to try and swim downward until you reach the bottom with various monsters and rock, as well as a depleting oxygen metre trying to prevent you. If you successfully make it to the bottom you will enter a new level without gravity and unlimited oxygen. This level is a level with lots of shallow water for neutral ground and some new monsters and npcs. Buildings offering new weapons and clothing will be availible as well as a new vehicle.
Wooden submarines
In order to leave this place you have to craft another ship or sub on the limited grass you can find. Once you build a ship you sail into a geisure ((thing shoots water upward)) and you will appear on surface.
There are many underwater caves and with at least 1 whirlpool per map kingdoms at war can take advantage of this by scouting them, reporting them and sneaking their fleet into the enemy kingdom waters.
Oh yea...Subs are the kingd of naval warfare as they can temporarily submerge to evade shipfire, do lots of damage against the ships and look cooler than ships..cannot carry players
It would be cool