There has been rampant speculation about the upcoming DLC, even mentions of a harpoon code have been revealed that weren't included in the original game content. Some believe that Bethesda may have intended some underwater gameplay in the original game, but later decided otherwise. There are two mysterious happenings that lead me to believe that this is actually going to be part of the DLC for the game;
At the Bunker Hill memorial village, it is possible as with anywhere else to eavesdrop on conversation. There, a conversation can be overheard regarding "a sea monster" discovered by fishermen, and that he attacked them on their boat. The description matches that of the mythical Kracken.
Secondly, there are random mutated dead sharks lying on beaches around the map, which at this point cannot be interacted with. However, as random as this sound, try shooting one and they will explode as any other NPC in the game is inclined to do. This suggests to me that the extent of the ocean gameplay is yet to be uncovered.
Watch this space!!!