The recent Fallout 4 update suggests that Fallout 4 DLC release date will be before the end of March 2016.
Fallout 4 released on November 10 and created a wave of appreciation since.
While the Fallout 4 DLC release date is getting closer, the content of the said version is not leaked by Bethesda. It is on the guessing stage depending on the previous experience. It may contain new missions, which the players will have to complete in order to receive rewards. Thrilling perks, weapons and armors may be part of the DLC version.
Underwater exploration can be a part of the new DLC content of Fallout 4. Fallout 4 update by researchers point at such a possibility. A Harpoon gun was found hidden in the code of the game. The gun fires out bubble apart from ammo. This indicates something related to water may be present in the version.
In one of the E3 images, one man was standing on the ocean floor. A first person perspective of ocean floor was also present. It is quite believable that the underground aspect will be there in the DLC version.
Other Fallout 4 update report indicates that Bethesda will be announcing more peaks once the first DLC version is out. However, the fans will have to wait for that till March this year. The company will also be announcing $30 Fallout 4 Season Pass quite soon.