The Tainted Sea is an area exclusive to Monster Hunter 3 Ultimate which is home only to the Elder DragonDire Miralis. The area consists of large pool of water and a land mass that each occupy roughly half of the area. The pool of water appears to be connected to the ocean and the Port Tanzia lighthouse can be seen in the distance.
In both corners of the shore there are two broken ships that hunters can climb onto. One ship contains a ballista mount, and the other ship contains one of the two dragonators. On the other side of the island in the pool of water is another ship that contains a second dragonator.
The base camp in the Tainted Sea can only be reached by using a Farcaster or by fainting. It contains a supply box, but there is no bed.
Savvy Tabby in Port Tanzia sometimes says that although Tainted Sea (which he also says lies near to Port Tanzia) has a scary name, it is an ordinary patch of sea and ships commonly pass through it without incident.